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LibWeb: Expand ClipFrame into clip rectangles during display list replay

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Until now, every paint phase of every PaintableBox injected its own
clipping sequence into the display list:
```
before_paint: Save
              AddClipRect (1)
              ...clip rectangles for each containing block with clip...
              AddClipRect (N)

paint:        ...paint phase items...

after_paint:  Restore
```

Because we ran that sequence for every phase of every box, Skia had to
rebuild clip stack `paint_phases * paintable_boxes` times. Worse,
usually most paint phases contribute no visible drawing at all, yet we
still had to emit clipping items because `before_paint()` has no way to
know that in advance.

This change takes a different approach:
- Clip information is now attached as metadata `ClipFrame` to each
  DisplayList item.
- `DisplayListPlayer` groups consecutive commands that share a
  `ClipFrame`, applying the clip once at the start of the group and
  restoring it once at the end.

Going from 10 ms to 5 ms in rasterization on Discord might not sound
like much, but keep in mind that for 60fps we have 16 ms per frame and
there is a lot more work besides display list rasterization we do in
each frame.

* https://discord.com/channels/1247070541085671459/1247090064480014443
  - DisplayList items:  81844  -> 3671
  - rasterize time:     10 ms  -> 5 ms
  - record time:        5 ms   -> 3 ms

* https://github.com/LadybirdBrowser/ladybird
  - DisplayList items:  7902  -> 1176
  - rasterize time:     4 ms  -> 4 ms
  - record time:        3 ms  -> 2 ms
This commit is contained in:
Aliaksandr Kalenik 2025-07-13 03:55:24 +02:00 • committed by Jelle Raaijmakers
commit eed47acb1f
Notes: github-actions[bot] 2025-07-14 13:49:39 +00:00
8 changed files with 104 additions and 72 deletions
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3
Libraries/LibWeb/Painting/PaintableBox.h
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@ -305,9 +305,6 @@ private:
bool scrollbar_contains_mouse_position(ScrollDirection, CSSPixelPoint);
void scroll_to_mouse_position(CSSPixelPoint);
static void apply_clip(PaintContext&, RefPtr<ClipFrame const> const&);
static void restore_clip(PaintContext&, RefPtr<ClipFrame const> const&);
OwnPtr<StackingContext> m_stacking_context;
Optional<OverflowData> m_overflow_data;