fix: improve line break preserving strategy

Author: VladaHarbour

packages/layout-engine/layout-bridge/src/remeasure.ts

@@ -1333,8 +1333,21 @@ export function remeasureParagraph(
 
     // Advance to next line start
     if (explicitLineBreakRun >= 0) {
-      // Explicit break consumed as a line boundary, continue from run after break.
-      currentRun = explicitLineBreakRun + 1;
+      // Preserve trailing/manual break boundaries:
+      // - If this line started on the break, we've already emitted its empty-line boundary,
+      //   so advance past it.
+      // - If this line ended before the break (text + break), only keep the break for the
+      //   next iteration when it is trailing or followed by another break. Otherwise consume
+      //   it immediately so mid-paragraph breaks don't create extra blank lines.
+      const emittedBreakBoundary =
+        startRun === explicitLineBreakRun && startChar === 0 && endRun === explicitLineBreakRun && endChar === 0;
+      if (emittedBreakBoundary) {
+        currentRun = explicitLineBreakRun + 1;
+      } else {
+        const nextRun = runs[explicitLineBreakRun + 1];
+        const preserveBoundaryForNextIteration = !nextRun || isLineBreakRun(nextRun);
+        currentRun = preserveBoundaryForNextIteration ? explicitLineBreakRun : explicitLineBreakRun + 1;
+      }
       currentChar = 0;
     } else {
       currentRun = endRun;
@@ -1343,7 +1356,7 @@ export function remeasureParagraph(
     if (currentRun >= runs.length) {
       break;
     }
-    if (currentChar >= runText(runs[currentRun]).length) {
+    if (!isLineBreakRun(runs[currentRun]) && currentChar >= runText(runs[currentRun]).length) {
       currentRun += 1;
       currentChar = 0;
     }

packages/layout-engine/layout-bridge/test/remeasure.test.ts

@@ -898,6 +898,34 @@ describe('remeasureParagraph', () => {
       expect(measure.lines[2].toRun).toBe(4);
     });
 
+    it('preserves trailing explicit lineBreak as final empty line', () => {
+      const block = createBlock([textRun('Hello'), { kind: 'lineBreak' } as Run]);
+      const measure = remeasureParagraph(block, 200);
+
+      expect(measure.lines).toHaveLength(2);
+      expect(measure.lines[0].fromRun).toBe(0);
+      expect(measure.lines[0].toRun).toBe(0);
+      // Final empty line should be anchored to trailing break run.
+      expect(measure.lines[1].fromRun).toBe(1);
+      expect(measure.lines[1].toRun).toBe(1);
+      expect(measure.lines[1].toChar).toBe(0);
+    });
+
+    it('preserves multiple trailing explicit lineBreak runs as multiple empty lines', () => {
+      const block = createBlock([textRun('Hello'), { kind: 'lineBreak' } as Run, { kind: 'lineBreak' } as Run]);
+      const measure = remeasureParagraph(block, 200);
+
+      expect(measure.lines).toHaveLength(3);
+      expect(measure.lines[0].fromRun).toBe(0);
+      expect(measure.lines[0].toRun).toBe(0);
+      expect(measure.lines[1].fromRun).toBe(1);
+      expect(measure.lines[1].toRun).toBe(1);
+      expect(measure.lines[1].toChar).toBe(0);
+      expect(measure.lines[2].fromRun).toBe(2);
+      expect(measure.lines[2].toRun).toBe(2);
+      expect(measure.lines[2].toChar).toBe(0);
+    });
+
     it('handles tabs followed immediately by line break', () => {
       const block = createBlock([textRun('A'), tabRun(), textRun('')]);
       const measure = remeasureParagraph(block, 200);