fix: render manual line breaks correctly

Author: VladaHarbour

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

@@ -1156,11 +1156,22 @@ export function remeasureParagraph(
     let endChar = currentChar;
     let tabStopCursor = 0;
     let didBreakInThisLine = false;
+    let explicitLineBreakRun = -1;
     let resumeRun = -1;
     let resumeChar = 0;
 
     for (let r = currentRun; r < runs.length; r += 1) {
       const run = runs[r];
+      if (isLineBreakRun(run)) {
+        explicitLineBreakRun = r;
+        if (startRun === r && startChar === 0 && width === 0) {
+          // Preserve leading/manual explicit break as an empty line.
+          endRun = r;
+          endChar = 0;
+        }
+        didBreakInThisLine = true;
+        break;
+      }
       if (run.kind === 'tab') {
         const absCurrentX = width + effectiveIndent;
         const { target, nextIndex, stop } = getNextTabStopPx(absCurrentX, tabStops, tabStopCursor);
@@ -1302,7 +1313,7 @@ export function remeasureParagraph(
     }
 
     // If we didn't consume any chars (e.g., very long single char), force one char
-    if (startRun === endRun && startChar === endChar) {
+    if (explicitLineBreakRun < 0 && startRun === endRun && startChar === endChar) {
       endRun = startRun;
       endChar = startChar + 1;
     }
@@ -1321,8 +1332,14 @@ export function remeasureParagraph(
     lines.push(line);
 
     // Advance to next line start
-    currentRun = endRun;
-    currentChar = endChar;
+    if (explicitLineBreakRun >= 0) {
+      // Explicit break consumed as a line boundary, continue from run after break.
+      currentRun = explicitLineBreakRun + 1;
+      currentChar = 0;
+    } else {
+      currentRun = endRun;
+      currentChar = endChar;
+    }
     if (currentRun >= runs.length) {
       break;
     }

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

@@ -872,7 +872,30 @@ describe('remeasureParagraph', () => {
       const block = createBlock([textRun('Hello'), { kind: 'lineBreak' } as Run, textRun('World')]);
       const measure = remeasureParagraph(block, 200);
 
-      expect(measure.lines.length).toBeGreaterThanOrEqual(1);
+      expect(measure.lines).toHaveLength(2);
+      expect(measure.lines[0].fromRun).toBe(0);
+      expect(measure.lines[0].toRun).toBe(0);
+      expect(measure.lines[1].fromRun).toBe(2);
+      expect(measure.lines[1].toRun).toBe(2);
+    });
+
+    it('preserves multiple explicit lineBreak boundaries', () => {
+      const block = createBlock([
+        textRun('One'),
+        { kind: 'lineBreak' } as Run,
+        textRun('Two'),
+        { kind: 'lineBreak' } as Run,
+        textRun('Three'),
+      ]);
+      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(2);
+      expect(measure.lines[1].toRun).toBe(2);
+      expect(measure.lines[2].fromRun).toBe(4);
+      expect(measure.lines[2].toRun).toBe(4);
     });
 
     it('handles tabs followed immediately by line break', () => {

packages/layout-engine/measuring/dom/src/index.test.ts

@@ -542,6 +542,8 @@ describe('measureBlock', () => {
       expect(measure.lines).toHaveLength(2);
       expect(measure.lines[0].width).toBeGreaterThan(0);
       expect(measure.lines[1].width).toBeGreaterThan(0);
+      expect(measure.lines[1].fromRun).toBe(2);
+      expect(measure.lines[1].toRun).toBe(2);
     });
 
     it('creates an empty line for leading lineBreak at start of paragraph', async () => {

packages/layout-engine/measuring/dom/src/index.ts

@@ -1397,6 +1397,21 @@ async function measureParagraphBlock(block: ParagraphBlock, maxWidth: number): P
       continue;
     }
 
+    // When a text/tab/atomic run follows an explicit lineBreak, currentLine is a
+    // placeholder line seeded with the break run index. Re-anchor it so line ranges
+    // start at the first visible run on the new line.
+    if (
+      currentLine &&
+      currentLine.width === 0 &&
+      currentLine.fromRun === currentLine.toRun &&
+      currentLine.fromChar === 0 &&
+      currentLine.toChar === 0 &&
+      isLineBreakRun(runsToProcess[currentLine.fromRun] as Run)
+    ) {
+      currentLine.fromRun = runIndex;
+      currentLine.toRun = runIndex;
+    }
+
     // Handle tab runs specially
     if (isTabRun(run)) {
       // Clear any previous tab group when we encounter a new tab