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script: Allow reflows that do not produce display lists (#37186)

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This change has two parts which depend on each other:

1. An early exit in the layout process, which allows for skipping
   display list construction entirely when nothing would change.
2. A simplification and unification of the way that "fake" animation
   frames are triggered. Now this happens on an entire ScriptThread at
   once and is based on whether or not any Pipeline triggered a display
   list update.

   Animations are never canceled in the compositor when the Pipeline
   isn't updating, instead the fake animation frame is triggered far
   enough in the future that an unexpected compositor tick will cancel
   it. This could happen, for instance, if some other Pipeline in some
   other ScriptThread produced a new display list for a tick. This makes
   everything simpler about these ticks.

The goal is that in a future change the ScriptThread-based animation
ticks will be made more generic so that they can throttle the number of
"update the rendering" calls triggered by script.

This should make Servo do a lot less work when moving the cursor over a
page. Before it would constantly produce new display lists.

Fixes: #17029.
Testing: This should not cause any web observable changes. The fact that
all WPT tests keep passing is the test for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
This commit is contained in:
Martin Robinson 2025-06-12 21:25:04 +02:00 • committed by GitHub
parent 29fc878e15
commit 23acb623c8
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GPG key ID: B5690EEEBB952194
11 changed files with 257 additions and 234 deletions
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64
components/layout/layout_impl.rs
View file

@ -136,8 +136,18 @@ pub struct LayoutThread {
/// A FontContext to be used during layout.
font_context: Arc<FontContext>,
/// Whether or not user agent stylesheets have been added to the Stylist or not.
have_added_user_agent_stylesheets: bool,
/// Is this the first reflow in this LayoutThread?
first_reflow: Cell<bool>,
have_ever_generated_display_list: Cell<bool>,
/// Whether a new display list is necessary due to changes to layout or stacking
/// contexts. This is set to true every time layout changes, even when a display list
/// isn't requested for this layout, such as for layout queries. The next time a
/// layout requests a display list, it is produced unconditionally, even when the
/// layout trees remain the same.
need_new_display_list: Cell<bool>,
/// The box tree.
box_tree: RefCell<Option<Arc<BoxTree>>>,
@ -520,7 +530,9 @@ impl LayoutThread {
registered_painters: RegisteredPaintersImpl(Default::default()),
image_cache: config.image_cache,
font_context: config.font_context,
first_reflow: Cell::new(true),
have_added_user_agent_stylesheets: false,
have_ever_generated_display_list: Cell::new(false),
need_new_display_list: Cell::new(false),
box_tree: Default::default(),
fragment_tree: Default::default(),
stacking_context_tree: Default::default(),
@ -659,9 +671,9 @@ impl LayoutThread {
);
self.calculate_overflow(damage);
self.build_stacking_context_tree(&reflow_request, damage);
self.build_display_list(&reflow_request, &mut layout_context);
let built_display_list =
self.build_display_list(&reflow_request, damage, &mut layout_context);
self.first_reflow.set(false);
if let ReflowGoal::UpdateScrollNode(scroll_state) = reflow_request.reflow_goal {
self.update_scroll_node_state(&scroll_state);
}
@ -674,6 +686,7 @@ impl LayoutThread {
std::mem::take(&mut *layout_context.node_image_animation_map.write());
Some(ReflowResult {
built_display_list,
pending_images,
pending_rasterization_images,
iframe_sizes,
@ -709,7 +722,7 @@ impl LayoutThread {
ua_stylesheets: &UserAgentStylesheets,
snapshot_map: &SnapshotMap,
) {
if self.first_reflow.get() {
if !self.have_added_user_agent_stylesheets {
for stylesheet in &ua_stylesheets.user_or_user_agent_stylesheets {
self.stylist
.append_stylesheet(stylesheet.clone(), guards.ua_or_user);
@ -726,6 +739,7 @@ impl LayoutThread {
guards.ua_or_user,
);
}
self.have_added_user_agent_stylesheets = true;
}
if reflow_request.stylesheets_changed {
@ -818,6 +832,9 @@ impl LayoutThread {
// had is now out of date and should be rebuilt.
*self.stacking_context_tree.borrow_mut() = None;
// Force display list generation as layout has changed.
self.need_new_display_list.set(true);
if self.debug.dump_style_tree {
println!(
"{:?}",
@ -879,28 +896,41 @@ impl LayoutThread {
fragment_tree,
viewport_size,
self.id.into(),
self.first_reflow.get(),
!self.have_ever_generated_display_list.get(),
&self.debug,
));
// Force display list generation as layout has changed.
self.need_new_display_list.set(true);
}
/// Build the display list for the current layout and send it to the renderer. If no display
/// list is built, returns false.
fn build_display_list(
&self,
reflow_request: &ReflowRequest,
damage: RestyleDamage,
layout_context: &mut LayoutContext<'_>,
) {
) -> bool {
if !reflow_request.reflow_goal.needs_display() {
return;
return false;
}
let Some(fragment_tree) = &*self.fragment_tree.borrow() else {
return;
return false;
};
let mut stacking_context_tree = self.stacking_context_tree.borrow_mut();
let Some(stacking_context_tree) = stacking_context_tree.as_mut() else {
return;
return false;
};
// It's not enough to simply check `damage` here as not all reflow requests
// require display lists. If a non-display-list-generating reflow updated layout
// in a previous refow, we cannot skip display list generation here the next time
// a display list is requested.
if !self.need_new_display_list.get() && !damage.contains(RestyleDamage::REPAINT) {
return false;
}
let mut epoch = self.epoch.get();
epoch.next();
self.epoch.set(epoch);
@ -922,7 +952,11 @@ impl LayoutThread {
.font_context
.collect_unused_webrender_resources(false /* all */);
self.compositor_api
.remove_unused_font_resources(keys, instance_keys)
.remove_unused_font_resources(keys, instance_keys);
self.have_ever_generated_display_list.set(true);
self.need_new_display_list.set(false);
true
}
fn update_scroll_node_state(&self, state: &ScrollState) {
@ -947,10 +981,10 @@ impl LayoutThread {
} else {
TimerMetadataFrameType::RootWindow
},
incremental: if self.first_reflow.get() {
TimerMetadataReflowType::FirstReflow
} else {
incremental: if self.have_ever_generated_display_list.get() {
TimerMetadataReflowType::Incremental
} else {
TimerMetadataReflowType::FirstReflow
},
})
}