LibWeb: Don't produce save & restore commands for scroll frame id change

This allows to substantially decrease display list length and
save/restore churn during painting.

Display list size comparison:

|               | Before | After  |
| ------------- | ------ | ------ |
| basecamp.com  | 19066  | 12248  |
| ladybird.org  | 8878   | 2220   |
| react.dev     | 39074  | 24382  |

Userland/Libraries/LibWeb/Painting/ClippableAndScrollable.cpp

@@ -47,12 +47,11 @@ void ClippableAndScrollable::apply_clip(PaintContext& context) const
 
     auto& display_list_recorder = context.display_list_recorder();
     display_list_recorder.save();
-    auto saved_scroll_frame_id = display_list_recorder.scroll_frame_id();
     for (auto const& clip_rect : clip_rects) {
         Optional<i32> clip_scroll_frame_id;
         if (clip_rect.enclosing_scroll_frame)
             clip_scroll_frame_id = clip_rect.enclosing_scroll_frame->id();
-        display_list_recorder.set_scroll_frame_id(clip_scroll_frame_id);
+        display_list_recorder.push_scroll_frame_id(clip_scroll_frame_id);
         auto rect = context.rounded_device_rect(clip_rect.rect).to_type<int>();
         auto corner_radii = clip_rect.corner_radii.as_corners(context);
         if (corner_radii.has_any_radius()) {
@@ -60,8 +59,8 @@ void ClippableAndScrollable::apply_clip(PaintContext& context) const
         } else {
             display_list_recorder.add_clip_rect(rect);
         }
+        display_list_recorder.pop_scroll_frame_id();
     }
-    display_list_recorder.set_scroll_frame_id(saved_scroll_frame_id);
 }
 
 void ClippableAndScrollable::restore_clip(PaintContext& context) const

Userland/Libraries/LibWeb/Painting/DisplayListRecorder.cpp

@@ -12,14 +12,16 @@ namespace Web::Painting {
 DisplayListRecorder::DisplayListRecorder(DisplayList& command_list)
     : m_command_list(command_list)
 {
-    m_state_stack.append(State());
 }
 
 DisplayListRecorder::~DisplayListRecorder() = default;
 
 void DisplayListRecorder::append(Command&& command)
 {
-    m_command_list.append(move(command), state().scroll_frame_id);
+    Optional<i32> scroll_frame_id;
+    if (!m_scroll_frame_id_stack.is_empty())
+        scroll_frame_id = m_scroll_frame_id_stack.last();
+    m_command_list.append(move(command), scroll_frame_id);
 }
 
 void DisplayListRecorder::paint_nested_display_list(RefPtr<DisplayList> display_list, Gfx::IntRect rect)
@@ -261,15 +263,21 @@ void DisplayListRecorder::translate(Gfx::IntPoint delta)
 void DisplayListRecorder::save()
 {
     append(Save {});
-    m_state_stack.append(m_state_stack.last());
 }
 
 void DisplayListRecorder::restore()
 {
     append(Restore {});
+}
 
-    VERIFY(m_state_stack.size() > 1);
-    m_state_stack.take_last();
+void DisplayListRecorder::push_scroll_frame_id(Optional<i32> id)
+{
+    m_scroll_frame_id_stack.append(id);
+}
+
+void DisplayListRecorder::pop_scroll_frame_id()
+{
+    (void)m_scroll_frame_id_stack.take_last();
 }
 
 void DisplayListRecorder::push_stacking_context(PushStackingContextParams params)
@@ -282,12 +290,12 @@ void DisplayListRecorder::push_stacking_context(PushStackingContextParams params
             .matrix = params.transform.matrix,
         },
         .clip_path = params.clip_path });
-    m_state_stack.append(State());
+    m_scroll_frame_id_stack.append({});
 }
 
 void DisplayListRecorder::pop_stacking_context()
 {
-    m_state_stack.take_last();
+    (void)m_scroll_frame_id_stack.take_last();
     append(PopStackingContext {});
 }
 

Userland/Libraries/LibWeb/Painting/DisplayListRecorder.h

@@ -103,15 +103,8 @@ public:
 
     void translate(Gfx::IntPoint delta);
 
-    void set_scroll_frame_id(Optional<i32> id)
-    {
-        state().scroll_frame_id = id;
-    }
-
-    Optional<i32> scroll_frame_id() const
-    {
-        return state().scroll_frame_id;
-    }
+    void push_scroll_frame_id(Optional<i32> id);
+    void pop_scroll_frame_id();
 
     void save();
     void restore();
@@ -157,13 +150,7 @@ public:
     void append(Command&& command);
 
 private:
-    struct State {
-        Optional<i32> scroll_frame_id;
-    };
-    State& state() { return m_state_stack.last(); }
-    State const& state() const { return m_state_stack.last(); }
-
-    Vector<State> m_state_stack;
+    Vector<Optional<i32>> m_scroll_frame_id_stack;
     DisplayList& m_command_list;
 };
 

Userland/Libraries/LibWeb/Painting/InlinePaintable.cpp

@@ -37,15 +37,15 @@ void InlinePaintable::before_paint(PaintContext& context, PaintPhase) const
     apply_clip(context);
 
     if (scroll_frame_id().has_value()) {
-        context.display_list_recorder().save();
-        context.display_list_recorder().set_scroll_frame_id(scroll_frame_id().value());
+        context.display_list_recorder().push_scroll_frame_id(scroll_frame_id().value());
     }
 }
 
 void InlinePaintable::after_paint(PaintContext& context, PaintPhase) const
 {
-    if (scroll_frame_id().has_value())
-        context.display_list_recorder().restore();
+    if (scroll_frame_id().has_value()) {
+        context.display_list_recorder().pop_scroll_frame_id();
+    }
 
     restore_clip(context);
 }

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -477,15 +477,15 @@ BorderRadiiData PaintableBox::normalized_border_radii_data(ShrinkRadiiForBorders
 void PaintableBox::apply_scroll_offset(PaintContext& context, PaintPhase) const
 {
     if (scroll_frame_id().has_value()) {
-        context.display_list_recorder().save();
-        context.display_list_recorder().set_scroll_frame_id(scroll_frame_id().value());
+        context.display_list_recorder().push_scroll_frame_id(scroll_frame_id().value());
     }
 }
 
 void PaintableBox::reset_scroll_offset(PaintContext& context, PaintPhase) const
 {
-    if (scroll_frame_id().has_value())
-        context.display_list_recorder().restore();
+    if (scroll_frame_id().has_value()) {
+        context.display_list_recorder().pop_scroll_frame_id();
+    }
 }
 
 void PaintableBox::apply_clip_overflow_rect(PaintContext& context, PaintPhase phase) const
@@ -687,7 +687,7 @@ void PaintableWithLines::paint(PaintContext& context, PaintPhase phase) const
         }
 
         if (own_scroll_frame_id().has_value()) {
-            context.display_list_recorder().set_scroll_frame_id(own_scroll_frame_id().value());
+            context.display_list_recorder().push_scroll_frame_id(own_scroll_frame_id().value());
         }
     }
 
@@ -716,6 +716,10 @@ void PaintableWithLines::paint(PaintContext& context, PaintPhase phase) const
 
     if (should_clip_overflow) {
         context.display_list_recorder().restore();
+
+        if (own_scroll_frame_id().has_value()) {
+            context.display_list_recorder().pop_scroll_frame_id();
+        }
     }
 }
 

Userland/Libraries/LibWeb/Painting/SVGSVGPaintable.cpp

@@ -31,8 +31,7 @@ void SVGSVGPaintable::before_children_paint(PaintContext& context, PaintPhase ph
     PaintableBox::before_children_paint(context, phase);
     if (phase != PaintPhase::Foreground)
         return;
-    context.display_list_recorder().save();
-    context.display_list_recorder().set_scroll_frame_id(scroll_frame_id());
+    context.display_list_recorder().push_scroll_frame_id(scroll_frame_id());
 }
 
 void SVGSVGPaintable::after_children_paint(PaintContext& context, PaintPhase phase) const
@@ -40,7 +39,7 @@ void SVGSVGPaintable::after_children_paint(PaintContext& context, PaintPhase pha
     PaintableBox::after_children_paint(context, phase);
     if (phase != PaintPhase::Foreground)
         return;
-    context.display_list_recorder().restore();
+    context.display_list_recorder().pop_scroll_frame_id();
 }
 
 static Gfx::FloatMatrix4x4 matrix_with_scaled_translation(Gfx::FloatMatrix4x4 matrix, float scale)

Userland/Libraries/LibWeb/Painting/StackingContext.cpp

@@ -335,8 +335,9 @@ void StackingContext::paint(PaintContext& context) const
     if (has_css_transform) {
         paintable_box().apply_clip_overflow_rect(context, PaintPhase::Foreground);
     }
-    if (paintable().is_paintable_box() && paintable_box().scroll_frame_id().has_value())
-        context.display_list_recorder().set_scroll_frame_id(*paintable_box().scroll_frame_id());
+    if (paintable().is_paintable_box() && paintable_box().scroll_frame_id().has_value()) {
+        context.display_list_recorder().push_scroll_frame_id(*paintable_box().scroll_frame_id());
+    }
     context.display_list_recorder().push_stacking_context(push_stacking_context_params);
 
     if (paintable().is_paintable_box()) {
@@ -353,6 +354,9 @@ void StackingContext::paint(PaintContext& context) const
 
     paint_internal(context);
     context.display_list_recorder().pop_stacking_context();
+    if (paintable().is_paintable_box() && paintable_box().scroll_frame_id().has_value()) {
+        context.display_list_recorder().pop_scroll_frame_id();
+    }
     if (has_css_transform)
         paintable_box().clear_clip_overflow_rect(context, PaintPhase::Foreground);
     context.display_list_recorder().restore();