LibWeb: Don't emit Push{Pop}StackingContext without visible effect

Before this change we would emit PushStackingContext/PopStackingContext
display list items regardless of whether the stacking context had any
transform/opacity/clip effects.

Display list size on https://x.com/ladybirdbrowser is reduced from ~2700
to ~800 items.

Libraries/LibGfx/Matrix.h

@@ -233,6 +233,18 @@ public:
         return determinant() != static_cast<T>(0.0);
     }
 
+    [[nodiscard]] bool is_identity() const
+    {
+        for (size_t i = 0; i < N; ++i) {
+            for (size_t j = 0; j < N; ++j) {
+                float expected = (i == j) ? 1.0f : 0.0f;
+                if ((*this)[i, j] != expected)
+                    return false;
+            }
+        }
+        return true;
+    }
+
 private:
     T m_elements[N][N];
 };

Libraries/LibWeb/Painting/DisplayListRecorder.h

@@ -35,6 +35,8 @@ struct StackingContextTransform {
     Gfx::FloatMatrix4x4 matrix;
 
     StackingContextTransform(Gfx::FloatPoint origin, Gfx::FloatMatrix4x4 matrix, float scale);
+
+    [[nodiscard]] bool is_identity() const { return matrix.is_identity(); }
 };
 
 class WEB_API DisplayListRecorder {
@@ -109,6 +111,8 @@ public:
         bool isolate;
         StackingContextTransform transform;
         Optional<Gfx::Path> clip_path = {};
+
+        bool has_effect() const { return opacity != 1.0f || compositing_and_blending_operator != Gfx::CompositingAndBlendingOperator::Normal || isolate || clip_path.has_value() || !transform.is_identity(); }
     };
     void push_stacking_context(PushStackingContextParams params);
     void pop_stacking_context();

Libraries/LibWeb/Painting/StackingContext.cpp

@@ -331,18 +331,24 @@ void StackingContext::paint(DisplayListRecordingContext& context) const
         paintable_box().apply_clip_overflow_rect(context, PaintPhase::Foreground);
     }
     paintable_box().apply_scroll_offset(context);
+
+    auto mask_image = computed_values.mask_image();
+    Optional<Gfx::Filter> resolved_filter;
+    if (computed_values.filter().has_filters())
+        resolved_filter = paintable_box().resolve_filter(computed_values.filter());
+
+    bool needs_to_save_state = mask_image || paintable_box().get_masking_area().has_value();
+
+    if (push_stacking_context_params.has_effect()) {
         context.display_list_recorder().push_stacking_context(push_stacking_context_params);
+    } else if (needs_to_save_state) {
+        context.display_list_recorder().save();
+    }
 
-    auto const& filter = computed_values.filter();
-    auto filter_applied = false;
-    if (filter.has_filters()) {
-        if (auto resolved_filter = paintable_box().resolve_filter(filter); resolved_filter.has_value()) {
+    if (resolved_filter.has_value())
         context.display_list_recorder().apply_filter(*resolved_filter);
-            filter_applied = true;
-        }
-    }
 
-    if (auto mask_image = computed_values.mask_image()) {
+    if (mask_image) {
         auto mask_display_list = DisplayList::create(context.device_pixels_per_css_pixel());
         DisplayListRecorder display_list_recorder(*mask_display_list);
         auto mask_painting_context = context.clone(display_list_recorder);
@@ -361,11 +367,14 @@ void StackingContext::paint(DisplayListRecordingContext& context) const
 
     paint_internal(context);
 
-    if (filter_applied) {
+    if (resolved_filter.has_value())
+        context.display_list_recorder().restore();
+
+    if (push_stacking_context_params.has_effect()) {
+        context.display_list_recorder().pop_stacking_context();
+    } else if (needs_to_save_state) {
         context.display_list_recorder().restore();
     }
-
-    context.display_list_recorder().pop_stacking_context();
     paintable_box().reset_scroll_offset(context);
     if (has_css_transform)
         paintable_box().clear_clip_overflow_rect(context, PaintPhase::Foreground);

Tests/LibWeb/Text/expected/css/text-underline-position.txt

@@ -1,12 +1,8 @@
 SaveLayer
-  PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
-    PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
   DrawGlyphRun rect=[35,8 8x18] translation=[35.15625,21.796875] color=rgb(0, 0, 0) scale=1
   DrawGlyphRun rect=[8,8 28x18] translation=[8,21.796875] color=rgb(0, 0, 0) scale=1
   DrawLine from=[8,24] to=[35,24] color=rgb(0, 0, 0) thickness=2
   DrawGlyphRun rect=[43,8 28x18] translation=[43.15625,21.796875] color=rgb(0, 0, 0) scale=1
   DrawLine from=[43,27] to=[71,27] color=rgb(0, 0, 0) thickness=2
-    PopStackingContext
-  PopStackingContext
 Restore
 

Tests/LibWeb/Text/expected/display_list/button-with-text-decoration.txt

@@ -1,11 +1,7 @@
 SaveLayer
-  PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
-    PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
   FillRect rect=[8,8 106x22] color=rgb(212, 208, 200)
   FillPath
   DrawGlyphRun rect=[13,10 96x18] translation=[13,23.796875] color=rgb(0, 0, 0) scale=1
   DrawLine from=[13,26] to=[109,26] color=rgb(0, 0, 0) thickness=2
-    PopStackingContext
-  PopStackingContext
 Restore
 

Tests/LibWeb/Text/expected/display_list/grid-gap-rounding.txt

@@ -1,12 +1,8 @@
 SaveLayer
-  PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
-    PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
   FillRect rect=[8,8 29x30] color=rgb(0, 0, 0)
   FillRect rect=[38,8 28x30] color=rgb(0, 0, 0)
   FillRect rect=[67,8 29x30] color=rgb(0, 0, 0)
   FillRect rect=[97,8 28x30] color=rgb(0, 0, 0)
   FillRect rect=[126,8 29x30] color=rgb(0, 0, 0)
-    PopStackingContext
-  PopStackingContext
 Restore
 

Tests/LibWeb/Text/expected/display_list/simple-overflow-hidden.txt

@@ -1,10 +1,6 @@
 SaveLayer
-  PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
-    PushStackingContext opacity=1 isolate=false has_clip_path=false transform=[1 0 0 1 0 0]
   FillPath
   FillRect rect=[10,10 300x150] color=rgb(240, 128, 128)
   DrawGlyphRun rect=[10,10 38x18] translation=[10,23.796875] color=rgb(0, 0, 0) scale=1
-    PopStackingContext
-  PopStackingContext
 Restore