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LibWeb: Apply scroll boxes offsets after painting commands recording

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With this change, instead of applying scroll offsets during the
recording of the painting command list, we do the following:
1. Collect all boxes with scrollable overflow into a PaintContext,
   each with an id and the total amount of scrolling offset accumulated
   from ancestor scrollable boxes.
2. During the recording phase assign a corresponding scroll_frame_id to
   each command that paints content within a scrollable box.
3. Before executing the recorded commands, translate each command that
   has a scroll_frame_id by the accumulated scroll offset.

This approach has following advantages:
- Implementing nested scrollables becomes much simpler, as the
  recording phase only requires the correct assignment of the nearest
  scrollable's scroll_frame_id, while the accumulated offset from
  ancestors is applied subsequently.
- The recording of painting commands is not tied to a specific offset
  within scrollable boxes, which means in the future, it will be
  possible to update the scrolling offset and repaint without the need
  to re-record painting commands.
This commit is contained in:
Aliaksandr Kalenik 2023-12-29 06:10:32 +01:00 • committed by Andreas Kling
parent d3025668a4
commit ac6b3c989d
12 changed files with 307 additions and 24 deletions
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19
Userland/Libraries/LibWeb/Painting/StackingContext.cpp
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@ -184,8 +184,16 @@ void StackingContext::paint_child(PaintContext& context, StackingContext const&
if (parent_paintable)
parent_paintable->before_children_paint(context, PaintPhase::Foreground);
PaintableBox const* nearest_scrollable_ancestor = child.paintable_box().nearest_scrollable_ancestor_within_stacking_context();
if (nearest_scrollable_ancestor)
nearest_scrollable_ancestor->apply_scroll_offset(context, PaintPhase::Foreground);
child.paint(context);
if (nearest_scrollable_ancestor)
nearest_scrollable_ancestor->reset_scroll_offset(context, PaintPhase::Foreground);
if (parent_paintable)
parent_paintable->after_children_paint(context, PaintPhase::Foreground);
}
@ -229,7 +237,7 @@ void StackingContext::paint_internal(PaintContext& context) const
// Apply scroll offset of nearest scrollable ancestor before painting the positioned descendant.
PaintableBox const* nearest_scrollable_ancestor = nullptr;
if (paintable.is_paintable_box())
nearest_scrollable_ancestor = static_cast<PaintableBox const&>(paintable).nearest_scrollable_ancestor();
nearest_scrollable_ancestor = static_cast<PaintableBox const&>(paintable).nearest_scrollable_ancestor_within_stacking_context();
if (nearest_scrollable_ancestor)
nearest_scrollable_ancestor->apply_scroll_offset(context, PaintPhase::Foreground);
@ -265,6 +273,12 @@ void StackingContext::paint_internal(PaintContext& context) const
for (auto* child : m_children) {
if (!child->paintable_box().is_positioned())
continue;
PaintableBox const* nearest_scrollable_ancestor = child->paintable_box().nearest_scrollable_ancestor_within_stacking_context();
if (nearest_scrollable_ancestor)
nearest_scrollable_ancestor->apply_scroll_offset(context, PaintPhase::Foreground);
auto containing_block = child->paintable_box().containing_block();
auto const* containing_block_paintable = containing_block ? containing_block->paintable() : nullptr;
if (containing_block_paintable)
@ -273,6 +287,9 @@ void StackingContext::paint_internal(PaintContext& context) const
paint_child(context, *child);
if (containing_block_paintable)
containing_block_paintable->clear_clip_overflow_rect(context, PaintPhase::Foreground);
if (nearest_scrollable_ancestor)
nearest_scrollable_ancestor->reset_scroll_offset(context, PaintPhase::Foreground);
}
paint_node(paintable_box(), context, PaintPhase::Outline);