fix(layout): guard the default tab-stop grid against a non-positive interval

packages/layout-engine/contracts/src/engines/tabs.test.ts

@@ -76,6 +76,29 @@ describe('engines-tabs computeTabStops', () => {
     expect(firstDefault?.source).toBe('default');
   });
 
+  it('generates no default grid and terminates when defaultTabInterval is 0 (#3944)', () => {
+    // Word writes <w:defaultTabStop w:val="0"/> when a user sets the default
+    // tab interval to 0 and opens such documents fine, with no automatic tab
+    // stops. Before the guard this looped forever, freezing the main thread.
+    const stops = computeTabStops({
+      explicitStops: [],
+      defaultTabInterval: 0,
+      paragraphIndent: { left: 0 },
+    });
+
+    expect(stops).toEqual([]);
+  });
+
+  it('keeps explicit stops but adds no default grid for a negative defaultTabInterval (#3944)', () => {
+    const stops = computeTabStops({
+      explicitStops: [{ val: 'start', pos: 720, leader: 'none' }],
+      defaultTabInterval: -10,
+      paragraphIndent: { left: 0 },
+    });
+
+    expect(stops).toEqual([{ val: 'start', pos: 720, leader: 'none', source: 'explicit' }]);
+  });
+
   it('adds an implicit left-margin stop when hanging indent starts before the margin', () => {
     const stops = computeTabStops({
       explicitStops: [{ val: 'start', pos: -1440, leader: 'none' }],

packages/layout-engine/contracts/src/engines/tabs.ts

@@ -186,28 +186,35 @@ export function computeTabStops(context: TabContext): TabStop[] {
   // - When no explicit start tabs exist (e.g., TOC paragraphs with only right-aligned tabs),
   //   seed defaults from the origin so numbering/content still lands on the default grid.
   // - Otherwise, preserve legacy behavior: defaults start after the rightmost explicit or left indent.
-  const seedDefaultsFromZero = !hasStartAlignedExplicit;
-  const defaultStart = seedDefaultsFromZero ? 0 : Math.max(maxExplicit, leftIndent);
-  let pos = defaultStart;
-  const targetLimit = Math.max(defaultStart, leftIndent, maxExplicit) + 14400; // 14400 twips = 10 inches
-
-  while (pos < targetLimit) {
-    pos += defaultTabInterval;
-
-    // Don't add if there's already a stop OR a cleared position at this position
-    const hasExistingStop = stops.some((s) => Math.abs(s.pos - pos) < TAB_POSITION_TOLERANCE_TWIPS);
-    const hasClearStop = clearPositions.some((clearPos) => Math.abs(clearPos - pos) < TAB_POSITION_TOLERANCE_TWIPS);
-
-    // Default stops must be >= leftIndent (for body text alignment)
-    const isValidDefault = pos >= leftIndent;
-
-    if (!hasExistingStop && !hasClearStop && isValidDefault) {
-      stops.push({
-        val: 'start',
-        pos,
-        leader: 'none',
-        source: 'default',
-      });
+  // w:defaultTabStop may be 0 or negative — Word writes 0 when a user sets the
+  // default tab interval to 0, opens such documents fine, and simply generates
+  // no automatic tab-stop grid. Without this guard the grid loop below never
+  // advances and freezes the main thread forever (issue #3944).
+  const hasUsableDefaultInterval = Number.isFinite(defaultTabInterval) && defaultTabInterval > 0;
+  if (hasUsableDefaultInterval) {
+    const seedDefaultsFromZero = !hasStartAlignedExplicit;
+    const defaultStart = seedDefaultsFromZero ? 0 : Math.max(maxExplicit, leftIndent);
+    let pos = defaultStart;
+    const targetLimit = Math.max(defaultStart, leftIndent, maxExplicit) + 14400; // 14400 twips = 10 inches
+
+    while (pos < targetLimit) {
+      pos += defaultTabInterval;
+
+      // Don't add if there's already a stop OR a cleared position at this position
+      const hasExistingStop = stops.some((s) => Math.abs(s.pos - pos) < TAB_POSITION_TOLERANCE_TWIPS);
+      const hasClearStop = clearPositions.some((clearPos) => Math.abs(clearPos - pos) < TAB_POSITION_TOLERANCE_TWIPS);
+
+      // Default stops must be >= leftIndent (for body text alignment)
+      const isValidDefault = pos >= leftIndent;
+
+      if (!hasExistingStop && !hasClearStop && isValidDefault) {
+        stops.push({
+          val: 'start',
+          pos,
+          leader: 'none',
+          source: 'default',
+        });
+      }
     }
   }