diff --git a/index.html b/index.html
index cac1154..0d24ca5 100644
--- a/index.html
+++ b/index.html
@@ -5,6 +5,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>MultiStack — MultiBoard Tile Stacker</title>
 <script src="https://cdnjs.cloudflare.com/ajax/libs/jszip/3.10.1/jszip.min.js"></script>
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 <style>
   @import url('https://fonts.googleapis.com/css2?family=Space+Mono:wght@400;700&family=Barlow+Condensed:wght@300;400;600;700;900&display=swap');
   :root {
@@ -48,6 +49,14 @@
   .file-info-icon { font-size:1.5rem; }
   .file-info-name { font-size:0.78rem; color:var(--success); word-break:break-all; }
   .file-info-meta { font-size:0.65rem; color:var(--muted); margin-top:3px; }
+  .file-list { display:grid; gap:8px; }
+  .file-row { display:flex; align-items:center; gap:14px; padding:12px 16px; background:rgba(61,220,132,0.05); border:1px solid rgba(61,220,132,0.2); flex-wrap:wrap; }
+  .file-row-name { font-size:0.75rem; color:var(--success); word-break:break-all; }
+  .file-row-meta { font-size:0.62rem; color:var(--muted); margin-top:2px; }
+  .file-row-count { display:flex; align-items:center; margin-left:auto; }
+  .file-row-count .sm-display { width:52px; }
+  .add-more { margin-top:10px; width:100%; padding:10px; background:transparent; border:1px dashed var(--border); color:var(--muted); font-family:var(--mono); font-size:0.68rem; letter-spacing:0.08em; text-transform:uppercase; cursor:pointer; transition:all 0.15s; }
+  .add-more:hover { border-color:var(--accent); color:var(--accent); }
   .file-clear { margin-left:auto; background:none; border:1px solid var(--border); color:var(--muted); cursor:pointer; padding:4px 10px; font-family:var(--mono); font-size:0.65rem; text-transform:uppercase; letter-spacing:0.08em; transition:all 0.15s; }
   .file-clear:hover { border-color:var(--accent); color:var(--accent); }
   .count-control { display:flex; align-items:center; }
@@ -60,8 +69,13 @@
   .row { display:flex; align-items:center; gap:24px; flex-wrap:wrap; margin-bottom:20px; }
   .gap-row { display:flex; align-items:center; gap:16px; flex-wrap:wrap; margin-top:4px; }
   .gap-label { font-size:0.68rem; color:var(--muted); text-transform:uppercase; letter-spacing:0.1em; }
+  .mode-row { display:flex; gap:8px; margin-bottom:18px; flex-wrap:wrap; }
+  .mode-btn { padding:10px 18px; background:var(--bg); border:1px solid var(--border); color:var(--muted); font-family:var(--mono); font-size:0.7rem; letter-spacing:0.08em; text-transform:uppercase; cursor:pointer; transition:all 0.15s; }
+  .mode-btn:hover { border-color:var(--accent); color:var(--accent); }
+  .mode-btn.active { border-color:var(--accent); color:var(--accent); background:rgba(255,107,43,0.08); }
   .sm-btn { width:36px; height:36px; background:var(--bg); border:1px solid var(--border); color:var(--text); font-size:1rem; cursor:pointer; font-family:var(--mono); transition:all 0.15s; }
   .sm-btn:hover { border-color:var(--accent); color:var(--accent); }
+  input.sm-display::-webkit-outer-spin-button, input.sm-display::-webkit-inner-spin-button { -webkit-appearance:none; margin:0; }
   .sm-display { width:72px; height:36px; background:var(--surface); border:1px solid var(--border); border-left:none; border-right:none; color:var(--accent); font-family:var(--display); font-size:1.1rem; font-weight:900; display:flex; align-items:center; justify-content:center; }
   .stack-preview { display:flex; align-items:flex-end; gap:4px; height:64px; margin-top:20px; }
   .preview-tile { width:32px; background:var(--accent); border:1px solid var(--accent2); border-radius:2px; box-shadow:0 0 8px rgba(255,107,43,0.4); transition:all 0.3s; }
@@ -118,24 +132,18 @@ <h1>Multi<span>Stack</span></h1>
     <div class="step active" id="step1">
       <div class="step-header">
         <div class="step-num">01</div>
-        <div class="step-title">Upload your tile STL</div>
+        <div class="step-title">Upload your tile STLs</div>
       </div>
       <div class="dropzone" id="dropzone">
-        <input type="file" id="fileInput" accept=".stl">
+        <input type="file" id="fileInput" accept=".stl" multiple>
         <span class="drop-icon">📦</span>
         <div class="drop-text">
           <strong>Click to browse</strong> or drag &amp; drop<br>
-          Your MultiBoard tile .stl file
+          One or more tile .stl files (tiles, corners, edges&hellip;)
         </div>
       </div>
-      <div class="file-info" id="fileInfo">
-        <span class="file-info-icon">&#x2705;</span>
-        <div>
-          <div class="file-info-name" id="fileName">&#x2014;</div>
-          <div class="file-info-meta" id="fileMeta">&#x2014;</div>
-        </div>
-        <button class="file-clear" onclick="clearFile()">Remove</button>
-      </div>
+      <div class="file-list" id="fileList"></div>
+      <button class="add-more" id="addMore" style="display:none" onclick="document.getElementById('fileInput').click()">+ Add more files</button>
     </div>
 
     <div class="step" id="step2">
@@ -143,24 +151,43 @@ <h1>Multi<span>Stack</span></h1>
         <div class="step-num">02</div>
         <div class="step-title">Choose stack settings</div>
       </div>
-      <div class="row">
-        <div class="count-control">
-          <button class="count-btn" id="btnMinus" onclick="adjustCount(-1)">&#x2212;</button>
-          <div class="count-display" id="countDisplay">2</div>
-          <button class="count-btn" id="btnPlus" onclick="adjustCount(1)">+</button>
-        </div>
-        <div class="count-label">tiles high<strong id="countLabel">2 tiles</strong></div>
+      <div class="mode-row">
+        <button class="mode-btn active" id="modeGap" onclick="setMode('gap')">Air gap</button>
+        <button class="mode-btn" id="modeIface" onclick="setMode('interface')">Interface layer (filament 2)</button>
       </div>
       <div class="gap-row">
-        <div class="gap-label">Inter-tile gap</div>
+        <div class="gap-label">Layer height</div>
+        <div style="display:flex;align-items:center">
+          <button class="sm-btn" onclick="adjustLayerH(-1)">&#x2212;</button>
+          <input class="sm-display" id="layerHDisplay" type="number" step="0.01" min="0.05" max="0.6" value="0.20" style="-moz-appearance:textfield">
+          <button class="sm-btn" onclick="adjustLayerH(1)">+</button>
+        </div>
+        <div style="font-size:0.68rem;color:var(--muted)">mm &nbsp;&mdash;&nbsp; must match the layer height you'll slice at (0.20 / 0.24 / 0.30&hellip;)</div>
+      </div>
+      <div class="gap-row" id="gapRow">
+        <div class="gap-label">Air gap layers</div>
         <div style="display:flex;align-items:center">
-          <button class="sm-btn" onclick="adjustGap(-1)">&#x2212;</button>
-          <div class="sm-display" id="gapDisplay">0.20</div>
-          <button class="sm-btn" onclick="adjustGap(1)">+</button>
+          <button class="sm-btn" onclick="adjustGapLayers(-1)">&#x2212;</button>
+          <div class="sm-display" id="gapLayersDisplay">1</div>
+          <button class="sm-btn" onclick="adjustGapLayers(1)">+</button>
         </div>
-        <div style="font-size:0.68rem;color:var(--muted)">mm &nbsp;&mdash;&nbsp; separation gap between tiles (default 0.20mm)</div>
+        <div style="font-size:0.68rem;color:var(--muted)">empty layers between tiles, snapped to the slicing grid &mdash; 1 is the classic stack-print gap</div>
+      </div>
+      <div class="gap-row" id="ifaceRow" style="display:none">
+        <div class="gap-label">Interface layers</div>
+        <div style="display:flex;align-items:center">
+          <button class="sm-btn" onclick="adjustIfaceLayers(-1)">&#x2212;</button>
+          <div class="sm-display" id="ifaceLayersDisplay">2</div>
+          <button class="sm-btn" onclick="adjustIfaceLayers(1)">+</button>
+        </div>
+        <div class="gap-label">Release gap</div>
+        <div style="display:flex;align-items:center">
+          <button class="sm-btn" onclick="adjustRelease(-1)">&#x2212;</button>
+          <div class="sm-display" id="releaseDisplay">0</div>
+          <button class="sm-btn" onclick="adjustRelease(1)">+</button>
+        </div>
+        <div style="font-size:0.68rem;color:var(--muted)">layer height must match what you'll slice at &mdash; interfaces are snapped to whole layers on the slicing grid. Release gap = air layers above each interface (like support top Z distance) so the tile printed on top peels off easily; 0 = direct contact, strongest bond</div>
       </div>
-      <div class="stack-preview" id="stackPreview"></div>
     </div>
 
     <div class="step" id="step3">
@@ -191,20 +218,21 @@ <h1>Multi<span>Stack</span></h1>
   </div>
 
   <div class="info-note">
-    <strong>Workflow:</strong> Upload your tile STL &rarr; choose stack count &rarr; download .3mf &rarr; open in BambuStudio &rarr; slice &rarr; send to printer.<br><br>
-    <strong>How stacking works:</strong> Each tile is placed directly on top of the previous one with a small gap (default 0.20mm). The printer deposits a single layer of air at each boundary, which is just enough for the tiles to pull apart cleanly when done.<br><br>
+    <strong>Workflow:</strong> Upload one or more tile STLs (tiles, corners, edges) &rarr; set each stack height &rarr; download .3mf &rarr; open in BambuStudio &rarr; slice &rarr; send to printer.<br><br>
+    <strong>How stacking works:</strong> <em>Air gap</em> places each tile on top of the previous one with a small gap (default 0.20mm) so a single layer of air separates them. <em>Interface layer</em> instead inserts a thin copy of the tile footprint between tiles, assigned to filament 2 &mdash; load a material that won't bond to your tiles (PETG between PLA tiles, or PLA between PETG tiles) and the stack pops apart cleanly with no fusing and no ironing tricks.<br><br>
     <strong>Tip:</strong> After opening in BambuStudio, check the plate looks correct before slicing. All tiles should be visible stacked vertically in the preview.
   </div>
 </div>
 
 <script>
 // ── State ────────────────────────────────────────────────────────────────────
-let uploadedFile = null;
-let stlData = null;       // ArrayBuffer of STL
-let stlInfo = null;       // { name, triangles, bbox, zHeight }
-let stackCount = 2;
-let gapSteps = 1;         // multiples of 0.20mm
+let files = [];           // [{ name, buf, info, count }]
+let sepMode = 'gap';      // 'gap' | 'interface'
+let ifaceLayers = 2;
+let releaseLayers = 0;
+let gapLayers = 1;
 let outputBlob = null;
+const MAX_STACK = 45;
 
 // ── UI helpers ────────────────────────────────────────────────────────────────
 function setProgress(pct, msg) {
@@ -226,39 +254,70 @@ <h1>Multi<span>Stack</span></h1>
   document.getElementById('progressArea').classList.remove('visible');
 }
 function updateStepStates() {
-  document.getElementById('step1').className = 'step ' + (uploadedFile ? 'done' : 'active');
-  document.getElementById('step2').className = 'step ' + (uploadedFile ? 'active' : '');
-  document.getElementById('step3').className = 'step ' + (uploadedFile ? 'active' : '');
-  document.getElementById('generateBtn').disabled = !uploadedFile;
+  const has = files.length > 0;
+  document.getElementById('step1').className = 'step ' + (has ? 'done' : 'active');
+  document.getElementById('step2').className = 'step ' + (has ? 'active' : '');
+  document.getElementById('step3').className = 'step ' + (has ? 'active' : '');
+  document.getElementById('generateBtn').disabled = !has;
+  document.getElementById('dropzone').style.display = has ? 'none' : '';
+  document.getElementById('addMore').style.display = has ? '' : 'none';
 }
-function buildStackPreview() {
-  const c = document.getElementById('stackPreview');
+function renderFileList() {
+  const c = document.getElementById('fileList');
   c.innerHTML = '';
-  const show = Math.min(stackCount, 10);
-  for (let i = 0; i < show; i++) {
-    const t = document.createElement('div');
-    t.className = 'preview-tile';
-    t.style.height = Math.min(12 + i*2, 56) + 'px';
-    c.appendChild(t);
-  }
-  if (stackCount > 10) {
-    const m = document.createElement('div');
-    m.style.cssText = 'font-size:0.6rem;color:var(--muted);margin-left:8px;align-self:center';
-    m.textContent = '+' + (stackCount-10) + ' more';
-    c.appendChild(m);
-  }
+  files.forEach((f, idx) => {
+    const row = document.createElement('div');
+    row.className = 'file-row';
+    row.innerHTML = `
+      <span class="file-info-icon">&#x2705;</span>
+      <div>
+        <div class="file-row-name">${f.name}</div>
+        <div class="file-row-meta">${f.info.triangles.toLocaleString()} tris &middot; ${(f.info.bbox.maxX-f.info.bbox.minX).toFixed(0)}&times;${(f.info.bbox.maxY-f.info.bbox.minY).toFixed(0)}&times;${f.info.zHeight.toFixed(1)} mm</div>
+      </div>
+      <div class="file-row-count">
+        <button class="sm-btn" onclick="adjustFileCount(${idx},-1)">&#x2212;</button>
+        <div class="sm-display">${f.count}</div>
+        <button class="sm-btn" onclick="adjustFileCount(${idx},1)">+</button>
+        <div style="font-size:0.62rem;color:var(--muted);margin-left:8px">high</div>
+      </div>
+      <button class="file-clear" style="margin-left:0" onclick="removeFile(${idx})">Remove</button>`;
+    c.appendChild(row);
+  });
+}
+function adjustFileCount(idx, d) {
+  files[idx].count = Math.max(1, Math.min(MAX_STACK, files[idx].count + d));
+  renderFileList();
+}
+function removeFile(idx) {
+  files.splice(idx, 1);
+  renderFileList();
+  updateStepStates();
+}
+function readMm(id) {
+  const v = parseFloat(document.getElementById(id).value);
+  return isNaN(v) ? 0.20 : Math.max(0.05, Math.min(1.2, v));
+}
+function adjustGapLayers(d) {
+  gapLayers = Math.max(1, Math.min(3, gapLayers + d));
+  document.getElementById('gapLayersDisplay').textContent = gapLayers;
+}
+function adjustLayerH(d) {
+  document.getElementById('layerHDisplay').value = (readMm('layerHDisplay') + d * 0.02).toFixed(2);
 }
-function adjustCount(d) {
-  stackCount = Math.max(2, Math.min(20, stackCount + d));
-  document.getElementById('countDisplay').textContent = stackCount;
-  document.getElementById('countLabel').textContent = stackCount + ' tiles';
-  document.getElementById('btnMinus').disabled = stackCount <= 2;
-  document.getElementById('btnPlus').disabled = stackCount >= 20;
-  buildStackPreview();
+function adjustIfaceLayers(d) {
+  ifaceLayers = Math.max(1, Math.min(3, ifaceLayers + d));
+  document.getElementById('ifaceLayersDisplay').textContent = ifaceLayers;
 }
-function adjustGap(d) {
-  gapSteps = Math.max(1, Math.min(5, gapSteps + d));
-  document.getElementById('gapDisplay').textContent = (gapSteps * 0.20).toFixed(2);
+function adjustRelease(d) {
+  releaseLayers = Math.max(0, Math.min(2, releaseLayers + d));
+  document.getElementById('releaseDisplay').textContent = releaseLayers;
+}
+function setMode(m) {
+  sepMode = m;
+  document.getElementById('modeGap').className = 'mode-btn' + (m==='gap' ? ' active' : '');
+  document.getElementById('modeIface').className = 'mode-btn' + (m==='interface' ? ' active' : '');
+  document.getElementById('gapRow').style.display = m==='gap' ? '' : 'none';
+  document.getElementById('ifaceRow').style.display = m==='interface' ? '' : 'none';
 }
 
 // ── File handling ─────────────────────────────────────────────────────────────
@@ -267,35 +326,28 @@ <h1>Multi<span>Stack</span></h1>
 dropzone.addEventListener('click', () => fileInput.click());
 dropzone.addEventListener('dragover', e => { e.preventDefault(); dropzone.classList.add('dragover'); });
 dropzone.addEventListener('dragleave', () => dropzone.classList.remove('dragover'));
-dropzone.addEventListener('drop', e => { e.preventDefault(); dropzone.classList.remove('dragover'); if(e.dataTransfer.files[0]) handleFile(e.dataTransfer.files[0]); });
-fileInput.addEventListener('change', e => { if(e.target.files[0]) handleFile(e.target.files[0]); });
+dropzone.addEventListener('drop', e => { e.preventDefault(); dropzone.classList.remove('dragover'); [...e.dataTransfer.files].forEach(handleFile); });
+fileInput.addEventListener('change', e => { [...e.target.files].forEach(handleFile); fileInput.value=''; });
 
 async function handleFile(f) {
-  if (!f.name.toLowerCase().endsWith('.stl')) { showError('Please upload a .stl file.'); return; }
-  uploadedFile = f;
-  stlData = await f.arrayBuffer();
-  stlInfo = parseSTLInfo(stlData);
-  document.getElementById('fileName').textContent = f.name;
-  document.getElementById('fileMeta').textContent =
-    `${stlInfo.triangles.toLocaleString()} triangles  ·  tile height: ${stlInfo.zHeight.toFixed(3)} mm  ·  ${(f.size/1024).toFixed(0)} KB`;
-  document.getElementById('dropzone').style.display = 'none';
-  document.getElementById('fileInfo').classList.add('visible');
+  if (!f.name.toLowerCase().endsWith('.stl')) { showError('Please upload .stl files only.'); return; }
+  const buf = await f.arrayBuffer();
+  const info = parseSTLInfo(buf);
+  files.push({ name: f.name, buf, info, count: 2 });
+  renderFileList();
   document.getElementById('errorArea').classList.remove('visible');
   document.getElementById('resultArea').classList.remove('visible');
   updateStepStates();
-  buildStackPreview();
 }
-function clearFile() {
-  uploadedFile = null; stlData = null; stlInfo = null;
+function resetAll() {
+  files = []; outputBlob = null;
   fileInput.value = '';
-  document.getElementById('dropzone').style.display = '';
-  document.getElementById('fileInfo').classList.remove('visible');
+  renderFileList();
   document.getElementById('errorArea').classList.remove('visible');
   document.getElementById('resultArea').classList.remove('visible');
   document.getElementById('progressArea').classList.remove('visible');
   updateStepStates();
 }
-function resetAll() { clearFile(); outputBlob = null; }
 
 // ── STL Parser ────────────────────────────────────────────────────────────────
 // Returns { triangles, zHeight, bbox:{minX,maxX,minY,maxY,minZ,maxZ}, isBinary }
@@ -355,7 +407,8 @@ <h1>Multi<span>Stack</span></h1>
 
 // ── STL → 3MF mesh XML ───────────────────────────────────────────────────────
 // Converts the STL binary data into 3MF <vertices> and <triangles> XML strings
-function stlToMeshXml(buf, isBinary) {
+function stlToMeshXml(buf, isBinary, zFn) {
+  const tz = zFn || (z => z);
   const vertices = [];
   const triangleIdxs = [];
   // We deduplicate vertices using a map for a smaller file
@@ -375,9 +428,9 @@ <h1>Multi<span>Stack</span></h1>
     const tCount = dv.getUint32(80, true);
     for (let i = 0; i < tCount; i++) {
       const base = 84 + i * 50;
-      const v0 = addVert(dv.getFloat32(base+12,true), dv.getFloat32(base+16,true), dv.getFloat32(base+20,true));
-      const v1 = addVert(dv.getFloat32(base+24,true), dv.getFloat32(base+28,true), dv.getFloat32(base+32,true));
-      const v2 = addVert(dv.getFloat32(base+36,true), dv.getFloat32(base+40,true), dv.getFloat32(base+44,true));
+      const v0 = addVert(dv.getFloat32(base+12,true), dv.getFloat32(base+16,true), tz(dv.getFloat32(base+20,true)));
+      const v1 = addVert(dv.getFloat32(base+24,true), dv.getFloat32(base+28,true), tz(dv.getFloat32(base+32,true)));
+      const v2 = addVert(dv.getFloat32(base+36,true), dv.getFloat32(base+40,true), tz(dv.getFloat32(base+44,true)));
       triangleIdxs.push([v0, v1, v2]);
     }
   } else {
@@ -385,9 +438,9 @@ <h1>Multi<span>Stack</span></h1>
     const verts = [...text.matchAll(/vertex\s+([\d.eE+\-]+)\s+([\d.eE+\-]+)\s+([\d.eE+\-]+)/g)];
     for (let i = 0; i < verts.length; i += 3) {
       if (i+2 >= verts.length) break;
-      const v0 = addVert(parseFloat(verts[i][1]),   parseFloat(verts[i][2]),   parseFloat(verts[i][3]));
-      const v1 = addVert(parseFloat(verts[i+1][1]), parseFloat(verts[i+1][2]), parseFloat(verts[i+1][3]));
-      const v2 = addVert(parseFloat(verts[i+2][1]), parseFloat(verts[i+2][2]), parseFloat(verts[i+2][3]));
+      const v0 = addVert(parseFloat(verts[i][1]),   parseFloat(verts[i][2]),   tz(parseFloat(verts[i][3])));
+      const v1 = addVert(parseFloat(verts[i+1][1]), parseFloat(verts[i+1][2]), tz(parseFloat(verts[i+1][3])));
+      const v2 = addVert(parseFloat(verts[i+2][1]), parseFloat(verts[i+2][2]), tz(parseFloat(verts[i+2][3])));
       triangleIdxs.push([v0, v1, v2]);
     }
   }
@@ -402,6 +455,176 @@ <h1>Multi<span>Stack</span></h1>
   return { vertsXml, trisXml, vertCount: vertices.length, triCount: triangleIdxs.length };
 }
 
+
+// ── Cross-section interface mesh ─────────────────────────────────────────────
+// Cuts the tile mesh with a horizontal plane at zRef, chains the intersection
+// segments into closed loops (outers + holes), and extrudes the resulting
+// footprint into a prism of the given thickness. This gives the interface the
+// tile's full mid-body outline at every sub-layer, instead of whatever the
+// squashed geometry happens to look like at the slice plane.
+function crossSectionMeshXml(buf, isBinary, zRef, thickness) {
+  // 1. gather triangles
+  const tris = [];
+  if (isBinary) {
+    const dv = new DataView(buf);
+    const n = dv.getUint32(80, true);
+    for (let i = 0; i < n; i++) {
+      const b = 84 + i*50;
+      tris.push([
+        [dv.getFloat32(b+12,true), dv.getFloat32(b+16,true), dv.getFloat32(b+20,true)],
+        [dv.getFloat32(b+24,true), dv.getFloat32(b+28,true), dv.getFloat32(b+32,true)],
+        [dv.getFloat32(b+36,true), dv.getFloat32(b+40,true), dv.getFloat32(b+44,true)],
+      ]);
+    }
+  } else {
+    const text = new TextDecoder().decode(buf);
+    const vs = [...text.matchAll(/vertex\s+([\d.eE+\-]+)\s+([\d.eE+\-]+)\s+([\d.eE+\-]+)/g)]
+      .map(m => [parseFloat(m[1]), parseFloat(m[2]), parseFloat(m[3])]);
+    for (let i = 0; i + 2 < vs.length; i += 3) tris.push([vs[i], vs[i+1], vs[i+2]]);
+  }
+
+  // avoid vertices exactly on the plane
+  zRef += 1.37e-4;
+
+  // 2. plane intersection → segments
+  const segs = [];
+  for (const t of tris) {
+    const above = t.map(v => v[2] > zRef);
+    if (above[0] === above[1] && above[1] === above[2]) continue;
+    const pts = [];
+    for (let e = 0; e < 3; e++) {
+      const a = t[e], b = t[(e+1)%3];
+      if ((a[2] > zRef) !== (b[2] > zRef)) {
+        const f = (zRef - a[2]) / (b[2] - a[2]);
+        pts.push([a[0] + f*(b[0]-a[0]), a[1] + f*(b[1]-a[1])]);
+      }
+    }
+    if (pts.length === 2) segs.push(pts);
+  }
+  if (!segs.length) throw new Error('Interface cross-section is empty — tile geometry not found at sample height.');
+
+  // 3. chain segments into closed loops (segments have arbitrary endpoint order,
+  // so index both endpoints and flip as needed while walking)
+  const key = p => Math.round(p[0]*5000) + ',' + Math.round(p[1]*5000);   // 0.2µm grid
+  const endMap = new Map();
+  const link = (k, i) => { if (!endMap.has(k)) endMap.set(k, []); endMap.get(k).push(i); };
+  segs.forEach((s, i) => { link(key(s[0]), i); link(key(s[1]), i); });
+  const used = new Array(segs.length).fill(false);
+  let loops = [];
+  for (let i = 0; i < segs.length; i++) {
+    if (used[i]) continue;
+    // drop degenerate (zero-length) segments
+    if (key(segs[i][0]) === key(segs[i][1])) { used[i] = true; continue; }
+    used[i] = true;
+    const loop = [segs[i][0], segs[i][1]];
+    const startKey = key(loop[0]);
+    let guard = segs.length + 8, closed = false;
+    while (guard-- > 0) {
+      const k = key(loop[loop.length-1]);
+      if (k === startKey) { closed = true; loop.pop(); break; }
+      const cands = (endMap.get(k) || []).filter(j => !used[j]);
+      if (!cands.length) break;
+      const j = cands[0];
+      used[j] = true;
+      const next = key(segs[j][0]) === k ? segs[j][1] : segs[j][0];   // take the far endpoint
+      if (key(next) === k) continue;   // degenerate, skip
+      loop.push(next);
+    }
+    if (!closed) {
+      // accept near-closed loops (float slop), discard open slivers
+      const a = loop[0], b = loop[loop.length-1];
+      const d2 = (a[0]-b[0])**2 + (a[1]-b[1])**2;
+      if (d2 > 0.01) continue;
+    }
+    if (loop.length >= 3) loops.push(loop);
+  }
+  if (!loops.length) throw new Error('Could not build closed interface contours.');
+
+  // 3b. simplify: drop collinear points (perp distance < 2µm) so walls and
+  // caps share identical vertices — keeps the prism watertight and small
+  for (let li = 0; li < loops.length; li++) {
+    const L = loops[li];
+    let out = L, changed = true;
+    while (changed && out.length > 3) {
+      changed = false;
+      const keep = [];
+      for (let i = 0; i < out.length; i++) {
+        const a = out[(i-1+out.length)%out.length], b = out[i], c = out[(i+1)%out.length];
+        const ux = c[0]-a[0], uy = c[1]-a[1];
+        const len = Math.hypot(ux, uy);
+        const perp = len < 1e-9 ? 0 : Math.abs(ux*(b[1]-a[1]) - uy*(b[0]-a[0])) / len;
+        const remaining = keep.length + (out.length - i - 1);   // if we drop b
+        if (perp < 0.002 && remaining >= 3) { changed = true; continue; }
+        keep.push(b);
+      }
+      if (keep.length >= 3) out = keep; else break;
+    }
+    loops[li] = out;
+  }
+  loops = loops.filter(L => L.length >= 3);
+  if (!loops.length) throw new Error('Interface contours degenerated during simplification.');
+
+  // 4. classify loops: containment depth → outer (even) / hole (odd)
+  const area = L => { let a=0; for (let i=0;i<L.length;i++){const p=L[i],q=L[(i+1)%L.length]; a += p[0]*q[1]-q[0]*p[1];} return a/2; };
+  const inside = (pt, L) => { let c=false; for (let i=0,j=L.length-1;i<L.length;j=i++){ const a=L[i],b=L[j];
+    if ((a[1]>pt[1])!==(b[1]>pt[1]) && pt[0] < (b[0]-a[0])*(pt[1]-a[1])/(b[1]-a[1])+a[0]) c=!c; } return c; };
+  const meta = loops.map(L => ({ L, area: area(L), depth: 0, parent: -1 }));
+  meta.forEach((m, i) => meta.forEach((n, j) => { if (i!==j && inside(m.L[0], n.L)) m.depth++; }));
+  meta.forEach((m) => {
+    if (m.depth % 2 === 1) {   // hole: find tightest containing outer
+      let best = -1, bestA = Infinity;
+      meta.forEach((n, j) => {
+        if (n.depth % 2 === 0 && inside(m.L[0], n.L) && Math.abs(n.area) < bestA) { best = j; bestA = Math.abs(n.area); }
+      });
+      m.parent = best;
+    }
+  });
+
+  // 5. build prism: walls for every loop, earcut caps per outer+its holes
+  const verts = [];
+  const trisOut = [];
+  const vmap = new Map();
+  function V(x, y, z) {
+    const k = x.toFixed(5)+','+y.toFixed(5)+','+z.toFixed(5);
+    if (vmap.has(k)) return vmap.get(k);
+    vmap.set(k, verts.length);
+    verts.push([x, y, z]);
+    return verts.length - 1;
+  }
+  // orient: outers CCW (area>0), holes CW
+  meta.forEach(m => {
+    const wantCCW = m.depth % 2 === 0;
+    if ((m.area > 0) !== wantCCW) m.L.reverse();
+  });
+  // walls
+  meta.forEach(m => {
+    const L = m.L;
+    for (let i = 0; i < L.length; i++) {
+      const p = L[i], q = L[(i+1)%L.length];
+      const p0 = V(p[0],p[1],0), q0 = V(q[0],q[1],0), q1 = V(q[0],q[1],thickness), p1 = V(p[0],p[1],thickness);
+      trisOut.push([p0,q0,q1],[p0,q1,p1]);
+    }
+  });
+  // caps
+  meta.forEach((m, mi) => {
+    if (m.depth % 2 !== 0) return;
+    const rings = [m.L, ...meta.filter(h => h.parent === mi).map(h => h.L)];
+    const flat = [], holeIdx = [];
+    rings.forEach((r, ri) => { if (ri > 0) holeIdx.push(flat.length/2); r.forEach(p => flat.push(p[0], p[1])); });
+    const idx = earcut.default(flat, holeIdx.length ? holeIdx : null);
+    const ringPts = rings.flat();
+    for (let i = 0; i < idx.length; i += 3) {
+      const a = ringPts[idx[i]], b = ringPts[idx[i+1]], c = ringPts[idx[i+2]];
+      trisOut.push([V(a[0],a[1],thickness), V(b[0],b[1],thickness), V(c[0],c[1],thickness)]);   // top
+      trisOut.push([V(a[0],a[1],0), V(c[0],c[1],0), V(b[0],b[1],0)]);                            // bottom (flipped)
+    }
+  });
+
+  const vertsXml = verts.map(([x,y,z]) => `      <vertex x="${x.toFixed(6)}" y="${y.toFixed(6)}" z="${z.toFixed(6)}"/>`).join('\n');
+  const trisXml = trisOut.map(([a,b,c]) => `      <triangle v1="${a}" v2="${b}" v3="${c}"/>`).join('\n');
+  return { vertsXml, trisXml, triCount: trisOut.length, loops: loops.length };
+}
+
 // ── Build BambuStudio project 3MF ────────────────────────────────────────────
 async function generate() {
   document.getElementById('errorArea').classList.remove('visible');
@@ -411,41 +634,96 @@ <h1>Multi<span>Stack</span></h1>
   document.getElementById('generateBtn').disabled = true;
 
   try {
-    setProgress(5, 'Parsing STL geometry…');
-    log(`File: ${uploadedFile.name}`);
-    log(`Triangles: ${stlInfo.triangles.toLocaleString()}`);
-    log(`Tile height: ${stlInfo.zHeight.toFixed(3)} mm`, 'ok');
-    log(`Bbox Z: ${stlInfo.bbox.minZ.toFixed(3)} → ${stlInfo.bbox.maxZ.toFixed(3)} mm`);
-
-    setProgress(20, 'Converting STL to 3MF mesh…');
-    const { vertsXml, trisXml, vertCount, triCount } = stlToMeshXml(stlData, stlInfo.isBinary);
-    log(`Mesh: ${vertCount} vertices, ${triCount} triangles`, 'ok');
-
-    const gapMm = gapSteps * 0.20;
-    const tileStep = stlInfo.zHeight + gapMm;  // vertical step per tile
-    // Z offset to bring tile bottom to z=0
-    const zFloor = stlInfo.bbox.minZ;
-    // Center tile on Bambu X1C bed (256x256mm)
-    const cx = 128 - (stlInfo.bbox.minX + stlInfo.bbox.maxX) / 2;
-    const cy = 128 - (stlInfo.bbox.minY + stlInfo.bbox.maxY) / 2;
-
-    log(`Gap: ${gapMm.toFixed(2)} mm between tiles`, 'ok');
-    log(`Stack: ${stackCount} tiles, step: ${tileStep.toFixed(3)} mm`);
-    log(`Centering: offset X=${cx.toFixed(2)} Y=${cy.toFixed(2)}`);
-
-    setProgress(40, 'Building 3MF model XML…');
-
-    // One <object> with the mesh, N <item> entries in <build> with Z-offset transforms
-    // 3MF transform: "r00 r01 r02 r10 r11 r12 r20 r21 r22 tx ty tz" (col-major rotation + translation)
-    // Identity rotation = "1 0 0 0 1 0 0 0 1 tx ty tz"
-    const buildItems = [];
-    for (let i = 0; i < stackCount; i++) {
-      const tz = i * tileStep - zFloor;  // shift bottom to 0, then step up
-      const tx = cx;
-      const ty = cy;
-      buildItems.push(`  <item objectid="1" transform="1 0 0 0 1 0 0 0 1 ${tx.toFixed(6)} ${ty.toFixed(6)} ${tz.toFixed(6)}" printable="1"/>`);
-    }
+    const layerH = readMm('layerHDisplay');
+    const gapMm = sepMode === 'gap' ? gapLayers * layerH : 0;
+    const ifaceMm = sepMode === 'interface' ? ifaceLayers * layerH : 0;
+    const releaseMm = sepMode === 'interface' ? releaseLayers * layerH : 0;
+
+    // ── Per-file meshes ──
+    const BED = 256, SPACING = 12;
+    const objectsXml = [];
+    const settingsXml = [];
+    const stacks = [];
+    let oid = 0, tallest = 0, totalTiles = 0;
+
+    files.forEach((f, fi) => {
+      setProgress(5 + 40*fi/files.length, `Converting ${f.name}\u2026`);
+      log(`${f.name}: ${f.info.triangles.toLocaleString()} tris, ${f.info.zHeight.toFixed(2)} mm tall, ${f.count} high`);
+      const tileName = f.name.replace('.stl','');
+      const tileId = ++oid;
+      const m = stlToMeshXml(f.buf, f.info.isBinary);
+      objectsXml.push(`    <object id="${tileId}" type="model" p:UUID="${randomUUID()}">\n      <mesh>\n        <vertices>\n${m.vertsXml}\n        </vertices>\n        <triangles>\n${m.trisXml}\n        </triangles>\n      </mesh>\n    </object>`);
+
+      // snap: everything above the tile top starts on the next slicing-grid line,
+      // so gaps/interfaces occupy exactly whole layers and every tile starts on grid
+      const pad = f.count > 1 ? Math.ceil(f.info.zHeight / layerH - 1e-6) * layerH - f.info.zHeight : 0;
+      if (pad > 1e-6) log(`${f.name}: tile height ${f.info.zHeight.toFixed(2)} isn't a multiple of ${layerH.toFixed(2)} — adding ${pad.toFixed(3)} mm to align tile boundaries to the layer grid`, 'warn');
+
+      let ifaceId = null;
+      if (sepMode === 'interface' && f.count > 1) {
+        ifaceId = ++oid;
+        const zRef = f.info.bbox.minZ + f.info.zHeight * 0.45;   // mid-body: full footprint
+        const im = crossSectionMeshXml(f.buf, f.info.isBinary, zRef, ifaceMm);
+        log(`${f.name}: interface footprint from cross-section at z=${(zRef - f.info.bbox.minZ).toFixed(2)} mm (${im.loops} contours)`, 'ok');
+        objectsXml.push(`    <object id="${ifaceId}" type="model" p:UUID="${randomUUID()}">\n      <mesh>\n        <vertices>\n${im.vertsXml}\n        </vertices>\n        <triangles>\n${im.trisXml}\n        </triangles>\n      </mesh>\n    </object>`);
+      }
 
+      // one native multi-part assembly per stack: components at fixed relative z.
+      // Its lowest point is z=0, so Studio's drop-to-plate can never shift parts,
+      // and no "multi-part object detected" dialog appears on import.
+      const step = f.info.zHeight + gapMm + pad + ifaceMm + releaseMm;
+      const asmId = ++oid;
+      const comps = [];
+      for (let i = 0; i < f.count; i++) {
+        comps.push(`        <component objectid="${tileId}" p:UUID="${randomUUID()}" transform="1 0 0 0 1 0 0 0 1 0 0 ${(i * step - f.info.bbox.minZ).toFixed(6)}"/>`);
+        if (ifaceId && i < f.count - 1) {
+          comps.push(`        <component objectid="${ifaceId}" p:UUID="${randomUUID()}" transform="1 0 0 0 1 0 0 0 1 0 0 ${(i * step + f.info.zHeight + pad).toFixed(6)}"/>`);
+        }
+      }
+      objectsXml.push(`    <object id="${asmId}" type="model" p:UUID="${randomUUID()}">\n      <components>\n${comps.join('\n')}\n      </components>\n    </object>`);
+
+      const ifacePart = ifaceId ? `\n    <part id="${ifaceId}" subtype="normal_part">\n      <metadata key="name" value="${tileName} interface"/>\n      <metadata key="extruder" value="2"/>\n      <metadata key="ironing_type" value="no ironing"/>\n      <metadata key="outer_wall_speed" value="80"/>\n      <metadata key="inner_wall_speed" value="80"/>\n      <metadata key="internal_solid_infill_speed" value="80"/>\n      <metadata key="top_surface_speed" value="80"/>\n    </part>` : '';
+      settingsXml.push(`  <object id="${asmId}">\n    <metadata key="name" value="${tileName} stack"/>\n    <metadata key="extruder" value="1"/>\n    <part id="${tileId}" subtype="normal_part">\n      <metadata key="name" value="${tileName}"/>\n      <metadata key="extruder" value="1"/>\n    </part>${ifacePart}\n  </object>`);
+
+      totalTiles += f.count;
+      tallest = Math.max(tallest, step * (f.count - 1) + f.info.zHeight);
+
+      stacks.push({
+        f, asmId,
+        w: f.info.bbox.maxX - f.info.bbox.minX,
+        d: f.info.bbox.maxY - f.info.bbox.minY,
+      });
+    });
+
+    // ── Shelf-arrange stacks on the plate ──
+    setProgress(50, 'Arranging stacks on plate\u2026');
+    let px = 0, py = 0, rowH = 0, maxW = 0;
+    stacks.forEach(s => {
+      if (px > 0 && px + s.w > BED) { px = 0; py += rowH + SPACING; rowH = 0; }
+      s.x = px; s.y = py;
+      px += s.w + SPACING;
+      rowH = Math.max(rowH, s.d);
+      maxW = Math.max(maxW, s.x + s.w);
+    });
+    const totalD = py + rowH;
+    if (maxW > BED || totalD > BED) log(`Arrangement is ${maxW.toFixed(0)}\u00d7${totalD.toFixed(0)} mm \u2014 may exceed your plate, check in BambuStudio`, 'warn');
+    const ox = (BED - maxW) / 2, oy = (BED - totalD) / 2;   // center the whole layout
+
+    // ── Build items: one per stack ──
+    const buildItems = [];
+    const assembleItems = [];
+    stacks.forEach(s => {
+      const info = s.f.info;
+      const cx = ox + s.x - info.bbox.minX;
+      const cy = oy + s.y - info.bbox.minY;
+      buildItems.push(`  <item objectid="${s.asmId}" transform="1 0 0 0 1 0 0 0 1 ${cx.toFixed(6)} ${cy.toFixed(6)} 0" printable="1"/>`);
+      assembleItems.push(`    <assemble_item object_id="${s.asmId}" instance_id="0" offset_to_bottom_mm="0"/>`);
+    });
+    if (sepMode === 'gap') log(`Air gap: ${gapLayers} layer(s) = ${gapMm.toFixed(2)} mm between tiles, on the slicing grid`, 'ok');
+    else log(`Interface: ${ifaceMm.toFixed(2)} mm in filament 2 between tiles${releaseMm > 0 ? ` + ${releaseMm.toFixed(2)} mm release gap above` : ''}`, 'ok');
+    log(`${files.length} stack(s), ${totalTiles} tiles, tallest ${tallest.toFixed(1)} mm`, 'ok');
+
+    setProgress(60, 'Building 3MF model XML\u2026');
     const modelXml = `<?xml version="1.0" encoding="UTF-8"?>
 <model unit="millimeter" xml:lang="en-US"
   xmlns="http://schemas.microsoft.com/3dmanufacturing/core/2015/02"
@@ -455,18 +733,9 @@ <h1>Multi<span>Stack</span></h1>
   <metadata name="Application">BambuStudio-02.02.02.56</metadata>
   <metadata name="BambuStudio:3mfVersion">1</metadata>
   <metadata name="CreationDate">${new Date().toISOString().slice(0,10)}</metadata>
-  <metadata name="Title">MultiStack - ${stackCount}x ${uploadedFile.name.replace('.stl','')}</metadata>
+  <metadata name="Title">MultiStack - ${files.length} stack(s)</metadata>
   <resources>
-    <object id="1" type="model" p:UUID="${randomUUID()}">
-      <mesh>
-        <vertices>
-${vertsXml}
-        </vertices>
-        <triangles>
-${trisXml}
-        </triangles>
-      </mesh>
-    </object>
+${objectsXml.join('\n')}
   </resources>
   <build>
 ${buildItems.join('\n')}
@@ -490,22 +759,16 @@ <h1>Multi<span>Stack</span></h1>
   <Relationship Target="/3D/3dmodel.model" Id="rel-1" Type="http://schemas.microsoft.com/3dmanufacturing/2013/01/3dmodel"/>
 </Relationships>`;
 
-    // Minimal model_settings with one entry per instance
-    const instanceSettings = Array.from({length: stackCount}, (_,i) => `
-  <object identify_id="${i+1}" id="${i+1}" name="${uploadedFile.name.replace('.stl','')} #${i+1}">
-    <metadata key="name" value="${uploadedFile.name.replace('.stl','')} #${i+1}"/>
-    <metadata key="extruder" value="1"/>
-  </object>`).join('');
-
     const modelSettings = `<?xml version="1.0" encoding="UTF-8"?>
 <config>
+${settingsXml.join('\n')}
   <plate>
     <metadata key="plater_id" value="1"/>
     <metadata key="plater_name" value=""/>
     <metadata key="locked" value="false"/>
   </plate>
   <assemble>
-    <assemble_item object_id="1" instance_id="0" offset_to_bottom_mm="${stlInfo.bbox.minZ.toFixed(6)}" />
+${assembleItems.join('\n')}
   </assemble>
 </config>`;
 
@@ -522,19 +785,18 @@ <h1>Multi<span>Stack</span></h1>
     log('Done!', 'ok');
     setProgress(100, 'Complete!');
 
-    const baseName = uploadedFile.name.replace(/\.stl$/i, '');
-    const outputName = `${baseName}_${stackCount}high.3mf`;
+    const baseName = files.length === 1 ? files[0].name.replace(/\.stl$/i, '') + `_${files[0].count}high` : `multistack_${files.length}stacks`;
+    const outputName = `${baseName}${sepMode==='interface' ? '_iface' : ''}.3mf`;
     const url = URL.createObjectURL(outputBlob);
     const dlBtn = document.getElementById('downloadBtn');
     dlBtn.href = url;
     dlBtn.download = outputName;
 
-    const totalH = (tileStep * (stackCount-1) + stlInfo.zHeight).toFixed(1);
     document.getElementById('resultStats').innerHTML = `
-      <div class="stat"><div class="stat-label">Tiles</div><div class="stat-value">${stackCount}</div></div>
-      <div class="stat"><div class="stat-label">Tile Height</div><div class="stat-value">${stlInfo.zHeight.toFixed(1)}<span class="stat-unit">mm</span></div></div>
-      <div class="stat"><div class="stat-label">Gap</div><div class="stat-value">${gapMm.toFixed(2)}<span class="stat-unit">mm</span></div></div>
-      <div class="stat"><div class="stat-label">Total Height</div><div class="stat-value">${totalH}<span class="stat-unit">mm</span></div></div>
+      <div class="stat"><div class="stat-label">Stacks</div><div class="stat-value">${files.length}</div></div>
+      <div class="stat"><div class="stat-label">Total Tiles</div><div class="stat-value">${totalTiles}</div></div>
+      <div class="stat"><div class="stat-label">${sepMode==='gap' ? 'Gap' : 'Interface'}</div><div class="stat-value">${(sepMode==='gap' ? gapMm : ifaceMm).toFixed(2)}<span class="stat-unit">mm</span></div></div>
+      <div class="stat"><div class="stat-label">Tallest</div><div class="stat-value">${tallest.toFixed(1)}<span class="stat-unit">mm</span></div></div>
       <div class="stat"><div class="stat-label">File Size</div><div class="stat-value">${(outputBlob.size/1024).toFixed(0)}<span class="stat-unit">KB</span></div></div>
     `;
     document.getElementById('resultArea').classList.add('visible');
@@ -556,7 +818,7 @@ <h1>Multi<span>Stack</span></h1>
 }
 
 // Init
-adjustCount(0);
+updateStepStates();
 </script>
 </body>
 </html>