LibWeb: Add indentation to display list dumps

Output display list dumps with an indentation level to show balanced
commands. It makes it much easier to see what is happening between e.g.
PushStackingContext and PopStackingContext, or SaveLayer and Restore.

Libraries/LibWeb/Painting/DisplayList.cpp

@@ -18,9 +18,28 @@ void DisplayList::append(DisplayListCommand&& command, Optional<i32> scroll_fram
 String DisplayList::dump() const
 {
     StringBuilder builder;
-    for (auto const& command : m_commands) {
+    int indentation = 0;
-        command.command.visit([&builder](auto const& cmd) { cmd.dump(builder); });
+    for (auto const& command_list_item : m_commands) {
-        builder.appendff("\n");
+        auto const& command = command_list_item.command;
+
+        command.visit([&indentation](auto const& command) {
+            if constexpr (requires { command.nesting_level_change; }) {
+                if (command.nesting_level_change < 0 && indentation >= -command.nesting_level_change)
+                    indentation += command.nesting_level_change;
+            }
+        });
+
+        if (indentation > 0)
+            builder.append(MUST(String::repeated("  "_string, indentation)));
+        command.visit([&builder](auto const& cmd) { cmd.dump(builder); });
+        builder.append('\n');
+
+        command.visit([&indentation](auto const& command) {
+            if constexpr (requires { command.nesting_level_change; }) {
+                if (command.nesting_level_change > 0)
+                    indentation += command.nesting_level_change;
+            }
+        });
     }
     return builder.to_string_without_validation();
 }

Libraries/LibWeb/Painting/DisplayListCommand.h

@@ -99,14 +99,20 @@ struct DrawRepeatedImmutableBitmap {
 };
 
 struct Save {
+    static constexpr int nesting_level_change = 1;
+
     void dump(StringBuilder&) const;
 };
 
 struct SaveLayer {
+    static constexpr int nesting_level_change = 1;
+
     void dump(StringBuilder&) const;
 };
 
 struct Restore {
+    static constexpr int nesting_level_change = -1;
+
     void dump(StringBuilder&) const;
 };
 
@@ -127,6 +133,8 @@ struct AddClipRect {
 };
 
 struct PushStackingContext {
+    static constexpr int nesting_level_change = 1;
+
     float opacity;
     Gfx::CompositingAndBlendingOperator compositing_and_blending_operator;
     bool isolate;
@@ -145,6 +153,8 @@ struct PushStackingContext {
 };
 
 struct PopStackingContext {
+    static constexpr int nesting_level_change = -1;
+
     void dump(StringBuilder&) const;
 };
 
@@ -445,16 +455,22 @@ struct PaintScrollBar {
 };
 
 struct ApplyOpacity {
+    static constexpr int nesting_level_change = 1;
+
     float opacity;
     void dump(StringBuilder&) const;
 };
 
 struct ApplyCompositeAndBlendingOperator {
+    static constexpr int nesting_level_change = 1;
+
     Gfx::CompositingAndBlendingOperator compositing_and_blending_operator;
     void dump(StringBuilder&) const;
 };
 
 struct ApplyFilter {
+    static constexpr int nesting_level_change = 1;
+
     Gfx::Filter filter;
     void dump(StringBuilder&) const;
 };

Libraries/LibWeb/Painting/DisplayListRecorder.cpp

@@ -462,7 +462,7 @@ void DisplayListRecorder::apply_opacity(float opacity)
 void DisplayListRecorder::apply_compositing_and_blending_operator(Gfx::CompositingAndBlendingOperator compositing_and_blending_operator)
 {
     // Implementation of this item does saveLayer(), so we need to increment the nesting level.
-    m_save_nesting_level++;
+    ++m_save_nesting_level;
     APPEND(ApplyCompositeAndBlendingOperator { .compositing_and_blending_operator = compositing_and_blending_operator });
 }