LibWeb: Cache flex item main sizes to avoid relayout during same cycle

This makes twitter.com actually load & render (although not very well.)

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp

@@ -619,7 +619,16 @@ void FlexFormattingContext::determine_flex_base_size_and_hypothetical_main_size(
         if (has_definite_main_size(child_box))
             return specified_main_size_of_child_box(child_box);
 
-        return calculate_indefinite_main_size(flex_item);
+        // NOTE: To avoid repeated layout work, we keep a cache of flex item main sizes on the
+        //       root FormattingState object. It's available through a full layout cycle.
+        // FIXME: Make sure this cache isn't overly permissive..
+        auto& size_cache = m_state.m_root.flex_item_size_cache;
+        auto it = size_cache.find(&flex_item.box);
+        if (it != size_cache.end())
+            return it->value;
+        auto main_size = calculate_indefinite_main_size(flex_item);
+        size_cache.set(&flex_item.box, main_size);
+        return main_size;
     }();
 
     // The hypothetical main size is the item’s flex base size clamped according to its used min and max main sizes (and flooring the content box size at zero).

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -107,6 +107,8 @@ struct FormattingState {
     };
     HashMap<NodeWithStyleAndBoxModelMetrics const*, IntrinsicSizes> mutable intrinsic_sizes;
 
+    HashMap<Box const*, float> mutable flex_item_size_cache;
+
     FormattingState const* m_parent { nullptr };
     FormattingState const& m_root;
 };