From dfc168033f9bf8feb59e1157d656e53765226440 Mon Sep 17 00:00:00 2001
From: BugFix <fix@primeng.org>
Date: Tue, 16 Jun 2026 08:24:44 +0000
Subject: [PATCH] fix(inputmask): prevent digit shift across section boundaries
 on delete

When using InputMask with a segmented pattern like '99.99.9999', pressing
Backspace or Delete inside one section would shift digits from subsequent
sections into the current one, causing values like '10.04.2026' to bleed
across the '.' separators.

Root cause: shiftL() iterated with seekNext() which skips over fixed mask
characters (null entries in the tests array). This allowed the left-shift
loop to pull digits across separator boundaries.

Fix: Added a countFixedChars() helper that counts null (fixed) positions in
a range. Before each shift step, we compare the number of fixed characters
encountered on the destination side (begin..i) vs. the source side
(end+1..j). If they differ, the shift has crossed a section boundary and
we break early.

The fix is applied to both the InputMask directive and the InputMask
standalone component classes.

Fixes #19629
---
 packages/primeng/src/inputmask/inputmask.ts | 36 +++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/packages/primeng/src/inputmask/inputmask.ts b/packages/primeng/src/inputmask/inputmask.ts
index 94fff6ea702..d74ff39060c 100755
--- a/packages/primeng/src/inputmask/inputmask.ts
+++ b/packages/primeng/src/inputmask/inputmask.ts
@@ -498,6 +498,18 @@ export class InputMaskDirective extends BaseComponent<InputMaskPassThrough> {
         return pos;
     }
 
+    /**
+     * Count the number of fixed (non-editable) mask characters in the range [from, to).
+     * Fixed characters are those where tests[k] is null (e.g. separators like '.' or '/').
+     */
+    private countFixedChars(from: number, to: number): number {
+        let count = 0;
+        for (let k = from; k < to; k++) {
+            if (!this.tests[k]) count++;
+        }
+        return count;
+    }
+
     shiftL(begin: number, end: number) {
         let i, j;
 
@@ -507,6 +519,12 @@ export class InputMaskDirective extends BaseComponent<InputMaskPassThrough> {
 
         for (i = begin, j = this.seekNext(end); i < (this.len as number); i++) {
             if (this.tests[i]) {
+                // Stop shifting if j has crossed more fixed-mask separators than i has.
+                // This prevents digits from bleeding across section boundaries (e.g. 99.99.9999).
+                if (this.countFixedChars(begin, i + 1) !== this.countFixedChars(end + 1, j + 1)) {
+                    break;
+                }
+
                 if (j < (this.len as number) && this.tests[i].test(this.buffer[j])) {
                     this.buffer[i] = this.buffer[j];
                     this.buffer[j] = this.getPlaceholder(j);
@@ -1070,6 +1088,18 @@ export class InputMask extends BaseInput<InputMaskPassThrough> {
         return pos;
     }
 
+    /**
+     * Count the number of fixed (non-editable) mask characters in the range [from, to).
+     * Fixed characters are those where tests[k] is null (e.g. separators like '.' or '/').
+     */
+    private countFixedChars(from: number, to: number): number {
+        let count = 0;
+        for (let k = from; k < to; k++) {
+            if (!this.tests[k]) count++;
+        }
+        return count;
+    }
+
     shiftL(begin: number, end: number) {
         let i, j;
 
@@ -1079,6 +1109,12 @@ export class InputMask extends BaseInput<InputMaskPassThrough> {
 
         for (i = begin, j = this.seekNext(end); i < (this.len as number); i++) {
             if (this.tests[i]) {
+                // Stop shifting if j has crossed more fixed-mask separators than i has.
+                // This prevents digits from bleeding across section boundaries (e.g. 99.99.9999).
+                if (this.countFixedChars(begin, i + 1) !== this.countFixedChars(end + 1, j + 1)) {
+                    break;
+                }
+
                 if (j < (this.len as number) && this.tests[i].test(this.buffer[j])) {
                     this.buffer[i] = this.buffer[j];
                     this.buffer[j] = this.getPlaceholder(j);