LibWeb: Factor out compute_y_position

Factor out the code that computes the vertical position of a Box with
respect to its siblings so that it can be used when computing the
absolutely positioned divs as well.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -505,57 +505,7 @@ void BlockFormattingContext::place_block_level_element_in_normal_flow_vertically
 
     compute_vertical_box_model_metrics(child_box, containing_block);
 
-    float y = box_state.border_box_top();
+    auto y = FormattingContext::compute_box_y_position_with_respect_to_siblings(child_box, box_state);
-
-    Vector<float> collapsible_margins;
-
-    auto* relevant_sibling = child_box.previous_sibling_of_type<Layout::BlockContainer>();
-    while (relevant_sibling != nullptr) {
-        if (!relevant_sibling->is_absolutely_positioned() && !relevant_sibling->is_floating()) {
-            auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
-            collapsible_margins.append(relevant_sibling_state.margin_bottom);
-            // NOTE: Empty (0-height) preceding siblings have their margins collapsed with *their* preceding sibling, etc.
-            if (relevant_sibling_state.border_box_height() > 0)
-                break;
-            collapsible_margins.append(relevant_sibling_state.margin_top);
-        }
-        relevant_sibling = relevant_sibling->previous_sibling_of_type<Layout::BlockContainer>();
-    }
-
-    if (relevant_sibling) {
-        // Collapse top margin with the collapsed margin(s) of preceding siblings.
-        collapsible_margins.append(box_state.margin_top);
-
-        float smallest_margin = 0;
-        float largest_margin = 0;
-        size_t negative_margin_count = 0;
-        for (auto margin : collapsible_margins) {
-            if (margin < 0)
-                ++negative_margin_count;
-            largest_margin = max(largest_margin, margin);
-            smallest_margin = min(smallest_margin, margin);
-        }
-
-        float collapsed_margin = 0;
-        if (negative_margin_count == collapsible_margins.size()) {
-            // When all margins are negative, the size of the collapsed margin is the smallest (most negative) margin.
-            collapsed_margin = smallest_margin;
-        } else if (negative_margin_count > 0) {
-            // When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.
-            collapsed_margin = largest_margin + smallest_margin;
-        } else {
-            // Otherwise, collapse all the adjacent margins by using only the largest one.
-            collapsed_margin = largest_margin;
-        }
-
-        auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
-        y += relevant_sibling_state.offset.y()
-            + relevant_sibling_state.content_height()
-            + relevant_sibling_state.border_box_bottom()
-            + collapsed_margin;
-    } else {
-        y += box_state.margin_top;
-    }
 
     auto clear_floating_boxes = [&](FloatSideData& float_side) {
         if (!float_side.current_boxes.is_empty()) {

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -1089,4 +1089,59 @@ float FormattingContext::containing_block_height_for(Box const& box, LayoutState
     VERIFY_NOT_REACHED();
 }
 
+float FormattingContext::compute_box_y_position_with_respect_to_siblings(Box const& child_box, LayoutState::UsedValues const& box_state)
+{
+    float y = box_state.border_box_top();
+
+    Vector<float> collapsible_margins;
+
+    auto* relevant_sibling = child_box.previous_sibling_of_type<Layout::BlockContainer>();
+    while (relevant_sibling != nullptr) {
+        if (!relevant_sibling->is_absolutely_positioned() && !relevant_sibling->is_floating()) {
+            auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
+            collapsible_margins.append(relevant_sibling_state.margin_bottom);
+            // NOTE: Empty (0-height) preceding siblings have their margins collapsed with *their* preceding sibling, etc.
+            if (relevant_sibling_state.border_box_height() > 0)
+                break;
+            collapsible_margins.append(relevant_sibling_state.margin_top);
+        }
+        relevant_sibling = relevant_sibling->previous_sibling_of_type<Layout::BlockContainer>();
+    }
+
+    if (relevant_sibling) {
+        // Collapse top margin with the collapsed margin(s) of preceding siblings.
+        collapsible_margins.append(box_state.margin_top);
+
+        float smallest_margin = 0;
+        float largest_margin = 0;
+        size_t negative_margin_count = 0;
+        for (auto margin : collapsible_margins) {
+            if (margin < 0)
+                ++negative_margin_count;
+            largest_margin = max(largest_margin, margin);
+            smallest_margin = min(smallest_margin, margin);
+        }
+
+        float collapsed_margin = 0;
+        if (negative_margin_count == collapsible_margins.size()) {
+            // When all margins are negative, the size of the collapsed margin is the smallest (most negative) margin.
+            collapsed_margin = smallest_margin;
+        } else if (negative_margin_count > 0) {
+            // When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.
+            collapsed_margin = largest_margin + smallest_margin;
+        } else {
+            // Otherwise, collapse all the adjacent margins by using only the largest one.
+            collapsed_margin = largest_margin;
+        }
+
+        auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
+        return y + relevant_sibling_state.offset.y()
+            + relevant_sibling_state.content_height()
+            + relevant_sibling_state.border_box_bottom()
+            + collapsed_margin;
+    } else {
+        return y + box_state.margin_top;
+    }
+}
+
 }

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -63,6 +63,8 @@ public:
 
     virtual void run_intrinsic_sizing(Box const&);
 
+    float compute_box_y_position_with_respect_to_siblings(Box const&, LayoutState::UsedValues const&);
+
 protected:
     FormattingContext(Type, LayoutState&, Box const&, FormattingContext* parent = nullptr);