LibWeb: Make flexbox special definite size cases explicit

The CSS-FLEXBOX-1 spec has a bunch of special cases where sizes are
considered definite after reaching a specific point of the layout
algorithm.

Before this change, we were relying on set_content_width/height also
implicitly marking those content sizes as definite.

To prepare for that implicit behavior going away, this patch makes
the special cases explicit.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp

@@ -83,6 +83,7 @@ void FlexFormattingContext::run(Box const& run_box, LayoutMode, AvailableSpace c
                 auto item_preferred_outer_cross_size = css_clamp(flex_container_inner_cross_size, item_min_cross_size, item_max_cross_size);
                 auto item_inner_cross_size = item_preferred_outer_cross_size - item.margins.cross_before - item.margins.cross_after - item.padding.cross_before - item.padding.cross_after - item.borders.cross_before - item.borders.cross_after;
                 set_cross_size(item.box, item_inner_cross_size);
+                set_has_definite_cross_size(item);
             }
         }
     }
@@ -153,6 +154,7 @@ void FlexFormattingContext::run(Box const& run_box, LayoutMode, AvailableSpace c
         if (flex_container_computed_cross_size.is_auto()) {
             for (auto& item : m_flex_items) {
                 set_cross_size(item.box, item.cross_size.value());
+                set_has_definite_cross_size(item);
             }
         }
     }
@@ -377,6 +379,22 @@ bool FlexFormattingContext::is_cross_auto(Box const& box) const
     return cross_length.is_auto();
 }
 
+void FlexFormattingContext::set_has_definite_main_size(FlexItem& item)
+{
+    if (is_row_layout())
+        item.used_values.set_has_definite_width(true);
+    else
+        item.used_values.set_has_definite_height(true);
+}
+
+void FlexFormattingContext::set_has_definite_cross_size(FlexItem& item)
+{
+    if (is_row_layout())
+        item.used_values.set_has_definite_height(true);
+    else
+        item.used_values.set_has_definite_width(true);
+}
+
 void FlexFormattingContext::set_main_size(Box const& box, CSSPixels size)
 {
     if (is_row_layout())
@@ -978,6 +996,12 @@ void FlexFormattingContext::resolve_flexible_lengths_for_line(FlexLine& line)
     for (auto& item : line.items) {
         item.main_size = item.target_main_size;
         set_main_size(item.box, item.target_main_size);
+
+        // https://drafts.csswg.org/css-flexbox-1/#definite-sizes
+        // 1. If the flex container has a definite main size, then the post-flexing main sizes of its flex items are treated as definite.
+        // 2. If a flex item’s flex basis is definite, then its post-flexing main size is also definite.
+        if (has_definite_main_size(m_flex_container_state) || item.used_flex_basis_is_definite)
+            set_has_definite_main_size(item);
     }
 }
 

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -159,6 +159,9 @@ private:
     void set_main_axis_first_margin(FlexItem&, CSSPixels margin);
     void set_main_axis_second_margin(FlexItem&, CSSPixels margin);
 
+    void set_has_definite_main_size(FlexItem&);
+    void set_has_definite_cross_size(FlexItem&);
+
     void copy_dimensions_from_flex_items_to_boxes();
 
     void generate_anonymous_flex_items();