fix(gui): the Windows title bar answers as the caption, so Snap works again (#6825)

docs/user/docs/windows.md

@@ -49,6 +49,7 @@ There is no built-in DAX audio driver on Windows. The DAX applet says so: "No bu
 - **Menus** are behind the **☰** button at the left of the title bar. Alt+letter mnemonics (Alt+F, Alt+S, …) still open each menu. F11 toggles full screen. See [Menu Reference](./menu-reference.md).
 - **Radio Setup** is **Settings → Radio Setup...**.
 - **Window frame:** a Qt-drawn frame with the system shadow and resize borders, with AetherSDR's own minimize, maximize and close buttons.
+- **Snap:** drag the title bar to a screen edge to snap the window to that half or corner, the same as any other window; Windows then offers Snap Assist to fill the rest of the screen. **Win+Left**, **Win+Right** and **Win+Up** snap it from the keyboard. Dragging works on the title bar's empty background — over the **☰** button, the radio tabs, the audio buttons and volume sliders, and the caption buttons a drag does what that control does instead. The logo and the labels that have their own hover text still move the window, but do not start a snap; drag from the empty background to snap. Hovering over the maximize button does not show the Snap Layouts flyout ([#6224](https://github.com/aethersdr/AetherSDR/issues/6224)).
 - **GPU choice on hybrid laptops:** the discrete GPU is used by default. The integrated GPU is listed in **Display → SYSTEM → GPU:** but grayed out ("disabled (#1921)"), because it crashes when a panadapter is popped out or docked. See [GPU Rendering](./gpu-rendering.md).
 - **Noise reduction:** **BNR** (NVIDIA Maxine, on an NVIDIA RTX/GeForce GPU) is available on Windows; its runtime comes as a single self-contained download. **MNR** is macOS only and is grayed out. See [BNR GPU Noise Removal](./bnr-gpu-noise-removal.md) and [DSP Noise Mitigation](./dsp-noise-mitigation.md).
 - **Copy Assist** can use a **Vulkan** GPU (NVIDIA, AMD or Intel). See [Copy Assist](./copy-assist.md).

src/gui/MainWindow.cpp

@@ -3876,6 +3876,7 @@ static LRESULT frameHitCode(WindowChrome::FrameHit hit)
     case WindowChrome::FrameHit::TopRight:    return HTTOPRIGHT;
     case WindowChrome::FrameHit::BottomLeft:  return HTBOTTOMLEFT;
     case WindowChrome::FrameHit::BottomRight: return HTBOTTOMRIGHT;
+    case WindowChrome::FrameHit::Caption:     return HTCAPTION;
     case WindowChrome::FrameHit::Client:      break;
     }
     return HTCLIENT;
@@ -3887,7 +3888,8 @@ bool MainWindow::nativeEvent(const QByteArray& eventType, void* message, qintptr
     // Under the expanded client area Qt 6.12 answers WM_NCHITTEST by turning
     // the live mouse-button state into synthetic presses, which doubles real
     // clicks and closes menus as they open (#6272). Answer it here instead:
-    // the resize band at the edges, client area everywhere else.
+    // the resize band at the edges, the title bar's own background as the
+    // caption so Windows offers Snap (#6825), client area everywhere else.
     if (msg && result && msg->message == WM_NCHITTEST
         && windowFlags().testFlag(Qt::ExpandedClientAreaHint)) {
         RECT window{};
@@ -3903,7 +3905,23 @@ bool MainWindow::nativeEvent(const QByteArray& eventType, void* message, qintptr
         const QPoint point(GET_X_LPARAM(msg->lParam), GET_Y_LPARAM(msg->lParam));
         const QRect rect(window.left, window.top,
                          window.right - window.left, window.bottom - window.top);
-        *result = frameHitCode(WindowChrome::expandedFrameHit(point, rect, borderX, borderY));
+        // The client area's top-left in screen pixels. GetWindowRect includes
+        // the invisible DWM resize border and Qt's geometry does not, so the
+        // caption gate anchors here instead — see logicalScreenPoint().
+        POINT clientOrigin{0, 0};
+        ClientToScreen(msg->hwnd, &clientOrigin);
+        // The bar's height is in logical pixels and WM_NCHITTEST is physical.
+        // Classify the message's point even when the shell queries it away
+        // from the cursor. Interactive controls stay client, as do tooltipped
+        // handles because caption mouse moves bypass Qt's tooltip timer.
+        const bool barShown = m_titleBar && m_titleBar->isVisible();
+        const int captionHeight = WindowChrome::captionHeightForHitTest(
+            point, window.top, QPoint(clientOrigin.x, clientOrigin.y),
+            mapToGlobal(QPoint(0, 0)), barShown ? m_titleBar->height() : 0,
+            devicePixelRatioF(),
+            [this](const QPoint& logical) { return m_titleBar->isCaptionMoveAreaAt(logical); });
+        *result = frameHitCode(
+            WindowChrome::expandedFrameHit(point, rect, borderX, borderY, captionHeight));
         return true;
     }
     // The whole window is client area, inset only while maximized so nothing

src/gui/TitleBar.cpp

@@ -14,6 +14,8 @@
 #include "core/NetworkDiagnostics.h"
 #include "core/AppSettings.h"
 
+#include <QAbstractButton>
+#include <QAbstractSlider>
 #include <QFrame>
 #include <QHBoxLayout>
 #include <QIcon>
@@ -841,6 +843,24 @@ bool TitleBar::isSystemMoveAreaAt(const QPoint& globalPos) const
     return false;
 }
 
+bool TitleBar::isCaptionMoveAreaAt(const QPoint& globalPos) const
+{
+    if (!isSystemMoveAreaAt(globalPos)) {
+        return false;
+    }
+    // Interactive children keep client input inside a draggable container.
+    // Tooltipped handles also stay client because caption mouse moves bypass
+    // Qt's tooltip timer.
+    for (QWidget* widget = childAt(mapFromGlobal(globalPos)); widget && widget != this;
+         widget = widget->parentWidget()) {
+        if (qobject_cast<QAbstractButton*>(widget) || qobject_cast<QAbstractSlider*>(widget)
+            || !widget->toolTip().isEmpty()) {
+            return false;
+        }
+    }
+    return true;
+}
+
 bool TitleBar::startWindowMove(QMouseEvent* ev)
 {
     if (!ev || ev->button() != Qt::LeftButton)

src/gui/TitleBar.h

@@ -122,6 +122,10 @@ class TitleBar : public QWidget {
 
     bool isSystemMoveAreaAt(const QPoint& globalPos) const;
 
+    // Windows caption subset: interactive controls and tooltipped handles
+    // stay client so their input and Qt tooltip delivery remain available.
+    bool isCaptionMoveAreaAt(const QPoint& globalPos) const;
+
     // ── Radio tabs ──────────────────────────────────────────────────────────
     // The strip of per-radio tabs plus the "+" discovered-radios popover.
     // MainWindow feeds these from discovery + the active connection.

src/gui/WindowChrome.h

@@ -90,27 +90,93 @@ inline bool claimsWholeWindowAsClient(Qt::WindowFlags flags, const QOperatingSys
 // live mouse-button state into synthetic presses, doubling real clicks (#6272).
 // Physical pixels; `borderX` / `borderY` are the resize bands on the left/right
 // and top/bottom edges (0 while maximized).
-enum class FrameHit { Client, Left, Right, Top, Bottom, TopLeft, TopRight, BottomLeft, BottomRight };
+//
+// `Caption` is the title bar's own draggable background: the answer Windows
+// needs before it will offer drag-to-edge Snap (#6825). Only the caller knows
+// which points are the bar's empty background rather than its tabs, menus and
+// buttons, so this reports Caption for the whole `captionHeight` strip and
+// MainWindow::nativeEvent narrows it with TitleBar::isCaptionMoveAreaAt() —
+// answering HTCAPTION over an interactive widget is what made menus close as
+// they opened (#6272). A `captionHeight` of 0 disables it, which is how every
+// non-main window keeps the old behaviour.
+enum class FrameHit { Client, Caption, Left, Right, Top, Bottom, TopLeft, TopRight, BottomLeft, BottomRight };
 
-inline FrameHit expandedFrameHit(const QPoint& point, const QRect& window, int borderX, int borderY)
+// WM_NCHITTEST carries the screen point being asked about in physical pixels,
+// while Qt's widget geometry — and so TitleBar::isSystemMoveAreaAt() — is in
+// logical ones.
+//
+// Anchor on the client origin: GetWindowRect includes Windows 11's
+// invisible DWM resize border, while Qt's widget geometry describes the
+// client area. Pairing the two client origins preserves control alignment.
+//
+// `physical` is the WM_NCHITTEST point; `clientOriginPhysical` is the client
+// area's top-left in screen pixels (GetClientRect + ClientToScreen);
+// `widgetOriginLogical` is that same corner as Qt places it
+// (QWidget::mapToGlobal({0, 0})). `dpr` is devicePixelRatioF().
+inline QPoint logicalScreenPoint(const QPoint& physical, const QPoint& clientOriginPhysical,
+                                 const QPoint& widgetOriginLogical, qreal dpr)
 {
-    if (borderX <= 0 || borderY <= 0 || !window.contains(point)) {
-        return FrameHit::Client;
+    if (!(dpr > 0)) {
+        return physical;
     }
-    const bool left = point.x() < window.left() + borderX;
-    const bool right = point.x() > window.right() - borderX;
-    const bool top = point.y() < window.top() + borderY;
-    const bool bottom = point.y() > window.bottom() - borderY;
-    if (left) {
-        return top ? FrameHit::TopLeft : bottom ? FrameHit::BottomLeft : FrameHit::Left;
+    return QPoint(widgetOriginLogical.x() + qRound((physical.x() - clientOriginPhysical.x()) / dpr),
+                  widgetOriginLogical.y() + qRound((physical.y() - clientOriginPhysical.y()) / dpr));
+}
+
+// Ask the move-area gate about the message's point in logical coordinates.
+// Measure from the client top, including the maximized frame inset. A missing
+// bar or rejected point gives no caption.
+//
+// `windowTop` is GetWindowRect's top; `clientOriginPhysical` the client's
+// top-left in screen pixels; `widgetOriginLogical` the same corner per Qt;
+// `barHeightLogical` the title bar's height; `dpr` devicePixelRatioF().
+template <typename MoveAreaTest>
+inline int captionHeightForHitTest(const QPoint& physical, int windowTop,
+                                   const QPoint& clientOriginPhysical,
+                                   const QPoint& widgetOriginLogical,
+                                   int barHeightLogical, qreal dpr,
+                                   MoveAreaTest&& isMoveArea)
+{
+    if (barHeightLogical <= 0) {
+        return 0;
+    }
+    if (!isMoveArea(logicalScreenPoint(physical, clientOriginPhysical,
+                                       widgetOriginLogical, dpr))) {
+        return 0;
+    }
+    return clientOriginPhysical.y() - windowTop + int(barHeightLogical * dpr);
+}
+
+inline FrameHit expandedFrameHit(const QPoint& point, const QRect& window, int borderX, int borderY,
+                                 int captionHeight = 0)
+{
+    if (!window.contains(point)) {
+        return FrameHit::Client;
     }
-    if (right) {
-        return top ? FrameHit::TopRight : bottom ? FrameHit::BottomRight : FrameHit::Right;
+    // Maximized and fullscreen have no resize band, but the caption still has
+    // to answer or the window cannot be dragged back off the top of the screen.
+    if (borderX > 0 && borderY > 0) {
+        const bool left = point.x() < window.left() + borderX;
+        const bool right = point.x() > window.right() - borderX;
+        const bool top = point.y() < window.top() + borderY;
+        const bool bottom = point.y() > window.bottom() - borderY;
+        if (left) {
+            return top ? FrameHit::TopLeft : bottom ? FrameHit::BottomLeft : FrameHit::Left;
+        }
+        if (right) {
+            return top ? FrameHit::TopRight : bottom ? FrameHit::BottomRight : FrameHit::Right;
+        }
+        if (top) {
+            return FrameHit::Top;
+        }
+        if (bottom) {
+            return FrameHit::Bottom;
+        }
     }
-    if (top) {
-        return FrameHit::Top;
+    if (captionHeight > 0 && point.y() < window.top() + captionHeight) {
+        return FrameHit::Caption;
     }
-    return bottom ? FrameHit::Bottom : FrameHit::Client;
+    return FrameHit::Client;
 }
 
 // Windows: the native window rect that gives `client`, keeping the frame the