perf(core): eliminate line-breaking bottleneck on massive unspaced text blocks (#28)

CHANGELOG.md

@@ -5,6 +5,7 @@
 - **Character-range highlight boxes:** `RenderHyperBox.getBoxesForCharRange(charStart, charEnd)` returns the rects of a character range, for drawing your own highlights (search hits, annotations, read-aloud). Offsets are local to each `RenderHyperBox`, so in virtualized / `auto` mode (>10k chars) they restart at 0 per chunk. Also new: `RenderHyperBox.debugLineFragments()` for the laid-out fragment geometry. By [@DrkXo](https://github.com/DrkXo) ([#26](https://github.com/brewkits/hyper_render/pull/26)).
 - **`text-overflow: ellipsis` fixes** (in `hyper_render_core`): the `…` glyph is no longer counted as a hidden source character, a selection highlight no longer covers it, and an empty line where nothing fits before the ellipsis now shows `…`.
 - **Line-breaking fixes** (in `hyper_render_core`, rendering changes): wrapped lines no longer overflow the box (every full CJK line overflowed by a fraction of a glyph), RTL paragraphs wrap properly, `word-break` / `overflow-wrap` set on a block reach its text (`break-all` had no effect), an over-long word breaks after as many characters as fit instead of after its first letter, a word that doesn't fit beside a float moves below it, lines no longer start one space in (after `<br>` or an indented tag), and centered / right-aligned lines ignore trailing spaces.
+- **Line-breaking performance on massive unspaced text blocks** ([#28](https://github.com/brewkits/hyper_render/issues/28), [#29](https://github.com/brewkits/hyper_render/pull/29)): single-pass native multi-line layout via ICU `computeLineMetrics()` when lines are uniform and free of floats, eliminating layout freezes on long unspaced text blocks. Bounded candidate prefix search in the fallback line breaker prevents quadratic `O(N^2)` text shaping overhead. New debug hooks `HyperRenderDebugHooks.onTextPainterLayout` / `onLineLayoutTextPainter` for layout call profiling. By [@DrkXo](https://github.com/DrkXo).
 - **Requires `hyper_render_core` `^1.13.0`.**
 
 ## 1.12.0

packages/hyper_render_core/CHANGELOG.md

@@ -14,6 +14,11 @@
   - **A word wider than the line breaks after as many characters as fit** (like Flutter's `Text`), instead of after its first letter, which left a column of one-letter lines.
   - **A word that doesn't fit beside a float moves below it** instead of being split after its first letter.
   - **Spaces at the start and end of a line collapse**, as in CSS. A line after `<p>` + newline, after `<br>`, or after an indented `<dt>` started one space in, and trailing spaces counted toward the width that `text-align: center/right` positions.
+- **Line-breaking performance on massive unspaced text blocks** ([#28](https://github.com/brewkits/hyper_render/issues/28), [#29](https://github.com/brewkits/hyper_render/pull/29)):
+  - Single-pass native multi-line layout fast path via ICU `computeLineMetrics()` when lines are uniform and free of floats, eliminating UI freezes on long unspaced text.
+  - Bounded candidate prefix search in the fallback line breaker loop to prevent quadratic `O(N^2)` HarfBuzz text shaping overhead.
+  - Robust loop termination on zero or negative line widths and trailing whitespace margin parity.
+- **`HyperRenderDebugHooks.onTextPainterLayout` / `onLineLayoutTextPainter`** — hook callbacks for counting text shaping and line layout operations in tests and DevTools. By [@DrkXo](https://github.com/DrkXo) ([#29](https://github.com/brewkits/hyper_render/pull/29)).
 
 ## 1.12.0
 

packages/hyper_render_core/lib/src/core/hyper_render_debug_hooks.dart

@@ -18,6 +18,8 @@ import '../model/node.dart';
 /// HyperRenderDebugHooks.onLayoutComplete   = (id, getFrags, getLines) { ... };
 /// HyperRenderDebugHooks.onFrameTiming      = (id, phase, micros) { ... };
 /// HyperRenderDebugHooks.onSelectionChanged = (id, start, end) { ... };
+/// HyperRenderDebugHooks.onTextPainterLayout = () { ... };
+/// HyperRenderDebugHooks.onLineLayoutTextPainter = () { ... };
 /// ```
 abstract final class HyperRenderDebugHooks {
   /// Called when a [RenderHyperBox] is attached to the pipeline.
@@ -63,6 +65,14 @@ abstract final class HyperRenderDebugHooks {
   /// selection was cleared.
   static void Function(String id, int? start, int? end)? onSelectionChanged;
 
+  /// Called in debug mode whenever a TextPainter performs layout in RenderHyperBox.
+  /// Used by tests and DevTools to count text shaping operations.
+  static void Function()? onTextPainterLayout;
+
+  /// Called in debug mode whenever a TextPainter performs layout during the line-breaking
+  /// pass in RenderHyperBox.
+  static void Function()? onLineLayoutTextPainter;
+
   /// CSS custom property overrides set from DevTools (`{'--name': value}`).
   ///
   /// In debug builds `HyperViewer` passes these to its
@@ -81,5 +91,7 @@ abstract final class HyperRenderDebugHooks {
           onRendererDetached != null ||
           onLayoutComplete != null ||
           onFrameTiming != null ||
-          onSelectionChanged != null);
+          onSelectionChanged != null ||
+          onTextPainterLayout != null ||
+          onLineLayoutTextPainter != null);
 }

packages/hyper_render_core/lib/src/core/render_hyper_box.dart

@@ -187,6 +187,10 @@ class RenderHyperBox extends RenderBox
   _LruCache<_TextPainterKey, TextPainter> get _textPainters =>
       _globalTextPainters;
 
+  /// Reusable TextPainter for measuring line-breaking candidate prefixes during
+  /// layout without polluting the global LRU cache.
+  TextPainter? _scratchCandidatePainter;
+
   /// Last collapsed margin (for margin collapsing between blocks)
   double _lastBlockMarginBottom = 0;
 
@@ -731,6 +735,8 @@ class RenderHyperBox extends RenderBox
     }
     _cancelBlockAnimationLoop();
     _disposeImages();
+    _scratchCandidatePainter?.dispose();
+    _scratchCandidatePainter = null;
     // Do NOT call detach() on cached semantic anchor nodes here.
     // Flutter's semantics teardown already detaches them during widget
     // unmounting — calling detach() again asserts inside SemanticsOwner
@@ -1088,6 +1094,7 @@ class RenderHyperBox extends RenderBox
 
         measurePainter.text = TextSpan(text: candidate, style: mergedStyle);
         measurePainter.layout();
+        if (kDebugMode) HyperRenderDebugHooks.onTextPainterLayout?.call();
         if (measurePainter.width > maxWidth) {
           maxWidth = measurePainter.width;
         }