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LibWeb: Skip effect contexts for off-screen display list commands

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When executing display list commands, check if commands with effect
contexts (opacity, filters, blend modes) are outside the viewport
before applying the effect. Since effects don't affect clip state,
would_be_fully_clipped_by_painter() returns the same result before
and after applying effects.

This avoids expensive saveLayer/restore cycles for off-screen commands
with effects like blur, which is particularly beneficial for pages with
many blurred decorative images (e.g., Discord's landing page has 70+
blurred star images).

The optimization only applies when switching to a new effect context,
not for consecutive commands with the same context, to preserve correct
blend mode compositing behavior.
This commit is contained in:
Aliaksandr Kalenik 2026-01-22 15:09:56 +01:00 committed by Alexander Kalenik
parent 26181f2958
commit cd0705334b
2 changed files with 17 additions and 1 deletions
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2
Libraries/LibWeb/Painting/AccumulatedVisualContext.h
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@ -86,6 +86,8 @@ public:
VisualContextData const& data() const { return m_data; }
RefPtr<AccumulatedVisualContext const> parent() const { return m_parent; }
bool is_effect() const { return m_data.has<EffectsData>(); }
size_t depth() const { return m_depth; }
size_t id() const { return m_id; }

16
Libraries/LibWeb/Painting/DisplayList.cpp
View file

@ -168,6 +168,21 @@ void DisplayListPlayer::execute_impl(DisplayList& display_list, ScrollStateSnaps
for (size_t command_index = 0; command_index < commands.size(); command_index++) {
auto const& [context, command] = commands[command_index];
auto bounding_rect = command_bounding_rectangle(command);
// OPTIMIZATION: If the leaf context is an effect and we're switching to a new context,
// check culling before applying it. Effects (opacity, filters, blend modes) don't affect
// clip state, so would_be_fully_clipped_by_painter() returns the same result before and after
// applying effects.
// This avoids expensive saveLayer/restore cycles for off-screen elements with effects like blur.
// NOTE: We must not do this for consecutive commands with the same context, as that would incorrectly restore
// and re-apply the effect layer, breaking blend mode compositing.
if (context && applied_context != context && context->is_effect() && bounding_rect.has_value()) {
switch_to_context(context->parent());
if (bounding_rect->is_empty() || would_be_fully_clipped_by_painter(*bounding_rect))
continue;
}
switch_to_context(context);
if (command.has<PaintScrollBar>()) {
@ -185,7 +200,6 @@ void DisplayListPlayer::execute_impl(DisplayList& display_list, ScrollStateSnaps
continue;
}
auto bounding_rect = command_bounding_rectangle(command);
if (bounding_rect.has_value() && (bounding_rect->is_empty() || would_be_fully_clipped_by_painter(*bounding_rect))) {
// Any clip or mask that's located outside of the visible region is equivalent to a simple clip-rect,
// so replace it with one to avoid doing unnecessary work.