LibWeb: Do not mutate display list while scroll offset application

Copy a display list item and apply scroll offset instead of mutating
display list directly.

It's a preparation for upcoming changes where a display list will be
cached across repaints and used multiple times with different scroll
offsets.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -2128,13 +2128,15 @@ RefPtr<Painting::DisplayList> Navigable::record_display_list(PaintConfig config)
 
     viewport_paintable.paint_all_phases(context);
 
-    Vector<Gfx::IntPoint> scroll_offsets_by_frame_id;
-    scroll_offsets_by_frame_id.resize(viewport_paintable.scroll_state.size());
-    for (auto [_, scrollable_frame] : viewport_paintable.scroll_state) {
-        auto scroll_offset = context.rounded_device_point(scrollable_frame->cumulative_offset).to_type<int>();
-        scroll_offsets_by_frame_id[scrollable_frame->id] = scroll_offset;
+    display_list->set_device_pixels_per_css_pixel(page.client().device_pixels_per_css_pixel());
+
+    Vector<RefPtr<Painting::ScrollFrame>> scroll_state;
+    scroll_state.resize(viewport_paintable.scroll_state.size());
+    for (auto& [_, scrollable_frame] : viewport_paintable.scroll_state) {
+        scroll_state[scrollable_frame->id] = scrollable_frame;
     }
-    display_list_recorder.display_list().apply_scroll_offsets(scroll_offsets_by_frame_id);
+
+    display_list->set_scroll_state(move(scroll_state));
 
     m_needs_repaint = false;
 

Userland/Libraries/LibWeb/Painting/DisplayList.cpp

@@ -24,28 +24,26 @@ static Optional<Gfx::IntRect> command_bounding_rectangle(Command const& command)
         });
 }
 
-void DisplayList::apply_scroll_offsets(Vector<Gfx::IntPoint> const& offsets_by_frame_id)
-{
-    for (auto& command_with_scroll_id : m_commands) {
-        if (command_with_scroll_id.scroll_frame_id.has_value()) {
-            auto const& scroll_frame_id = command_with_scroll_id.scroll_frame_id.value();
-            auto const& scroll_offset = offsets_by_frame_id[scroll_frame_id];
-            command_with_scroll_id.command.visit(
-                [&](auto& command) {
-                    if constexpr (requires { command.translate_by(scroll_offset); })
-                        command.translate_by(scroll_offset);
-                });
-        }
-    }
-}
-
 void DisplayListPlayer::execute(DisplayList& display_list)
 {
     auto const& commands = display_list.commands();
+    auto const& scroll_state = display_list.scroll_state();
+    auto device_pixels_per_css_pixel = display_list.device_pixels_per_css_pixel();
 
     size_t next_command_index = 0;
     while (next_command_index < commands.size()) {
-        auto const& command = commands[next_command_index++].command;
+        auto scroll_frame_id = commands[next_command_index].scroll_frame_id;
+        auto command = commands[next_command_index++].command;
+        if (scroll_frame_id.has_value()) {
+            auto const& scroll_offset = scroll_state[scroll_frame_id.value()]->cumulative_offset.to_type<double>().scaled(device_pixels_per_css_pixel).to_type<int>();
+            command.visit(
+                [&](auto& command) {
+                    if constexpr (requires { command.translate_by(scroll_offset); }) {
+                        command.translate_by(scroll_offset);
+                    }
+                });
+        }
+
         auto bounding_rect = command_bounding_rectangle(command);
         if (bounding_rect.has_value() && (bounding_rect->is_empty() || would_be_fully_clipped_by_painter(*bounding_rect))) {
             continue;

Userland/Libraries/LibWeb/Painting/DisplayList.h

@@ -29,6 +29,7 @@
 #include <LibWeb/Painting/Command.h>
 #include <LibWeb/Painting/GradientData.h>
 #include <LibWeb/Painting/PaintBoxShadowParams.h>
+#include <LibWeb/Painting/ScrollFrame.h>
 
 namespace Web::Painting {
 
@@ -83,8 +84,6 @@ public:
 
     void append(Command&& command, Optional<i32> scroll_frame_id);
 
-    void apply_scroll_offsets(Vector<Gfx::IntPoint> const& offsets_by_frame_id);
-
     struct CommandListItem {
         Optional<i32> scroll_frame_id;
         Command command;
@@ -92,10 +91,19 @@ public:
 
     AK::SegmentedVector<CommandListItem, 512> const& commands() const { return m_commands; }
 
+    void set_scroll_state(Vector<RefPtr<ScrollFrame>> scroll_state) { m_scroll_state = move(scroll_state); }
+
+    Vector<RefPtr<ScrollFrame>> const& scroll_state() const { return m_scroll_state; }
+
+    void set_device_pixels_per_css_pixel(double device_pixels_per_css_pixel) { m_device_pixels_per_css_pixel = device_pixels_per_css_pixel; }
+    double device_pixels_per_css_pixel() const { return m_device_pixels_per_css_pixel; }
+
 private:
     DisplayList() = default;
 
     AK::SegmentedVector<CommandListItem, 512> m_commands;
+    Vector<RefPtr<ScrollFrame>> m_scroll_state;
+    double m_device_pixels_per_css_pixel;
 };
 
 }