perf(text): fast path for single-space separators in wrapLine + tests for #128 (#130)

PR #128 made wrapLine measure the real separator string so multi-space
runs are counted at their true width, but that added a measureText call
per word gap even for the dominant single ' ' case — on the Canvas
backend that's a ctx.measureText per gap during layout. Reuse the
precomputed spaceWidth when the separator is exactly one space and only
measure multi-char runs.

Also adds the unit tests #128 landed without: multi-space separators
wrap at their real width and are preserved when the line fits, ZWSP
runs stay zero-width, and the single-space path uses the precomputed
width.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: chiefcll

src/core/text-rendering/TextLayoutEngine.ts

@@ -461,9 +461,14 @@ export const wrapLine = (
       continue;
     }
     const space = spaces[spaceIdx++] || '';
-    // Measure the real separator \u2014 multi-space runs collapse into one match.
+    // Single ' ' is the dominant separator \u2014 reuse the precomputed width.
+    // Multi-space/ZWSP runs collapse into one match; measure those for real.
     const effectiveSpaceWidth =
-      space.length > 0 ? measureText(space, fontFamily, letterSpacing) : 0;
+      space === ' '
+        ? spaceWidth
+        : space.length > 0
+        ? measureText(space, fontFamily, letterSpacing)
+        : 0;
     const totalWidth = currentLineWidth + effectiveSpaceWidth + wordWidth;
 
     if (totalWidth < maxWidth) {

src/core/text-rendering/tests/TextLayoutEngine.test.ts

@@ -270,6 +270,83 @@ describe('SDF Text Utils', () => {
       expect(lines).toHaveLength(1);
       expect(lines[0]?.[0]).toBe('   hello world');
     });
+
+    it('should count the full width of a multi-space separator when wrapping', () => {
+      // 'aa  bb' really measures 60 (6 chars * 10). Counting the separator
+      // as a single spaceWidth (50) used to keep it on one line past maxWidth.
+      const result = wrapLine(
+        testMeasureText,
+        'aa  bb',
+        'Arial',
+        55, // maxWidth — below the real 60, above the buggy 50
+        0,
+        10, // spaceWidth
+        '',
+        0,
+        'break-word',
+        10,
+      );
+
+      const [lines] = result;
+      expect(lines).toHaveLength(2);
+      expect(lines[0]).toEqual(['aa', 20, false, 0, 0]);
+      expect(lines[1]).toEqual(['bb', 20, false, 0, 0]);
+    });
+
+    it('should preserve a multi-space separator and its width when the line fits', () => {
+      const result = wrapLine(
+        testMeasureText,
+        'aa  bb',
+        'Arial',
+        1000,
+        0,
+        10, // spaceWidth
+        '',
+        0,
+        'break-word',
+        10,
+      );
+
+      expect(result[0][0]).toEqual(['aa  bb', 60, false, 0, 0]);
+    });
+
+    it('should treat a run of multiple ZWSPs as a zero-width separator', () => {
+      const result = wrapLine(
+        testMeasureText,
+        'aa\u200B\u200Bbb',
+        'Arial',
+        1000,
+        0,
+        10, // spaceWidth
+        '',
+        0,
+        'break-word',
+        10,
+      );
+
+      // Zero-width separator: no visible gap added, width is just the glyphs
+      expect(result[0][0]).toEqual(['aabb', 40, false, 0, 0]);
+    });
+
+    it('should use the precomputed spaceWidth for a single-space separator', () => {
+      // spaceWidth (4) deliberately differs from testMeasureText(' ') (10):
+      // the single-space fast path must reuse the precomputed value instead
+      // of re-measuring the separator.
+      const result = wrapLine(
+        testMeasureText,
+        'aa bb',
+        'Arial',
+        1000,
+        0,
+        4, // spaceWidth
+        '',
+        0,
+        'break-word',
+        10,
+      );
+
+      expect(result[0][0]).toEqual(['aa bb', 44, false, 0, 0]);
+    });
   });
 
   describe('wrapText', () => {