layout: Unify scrollable overflow calculation and include position: absolute (#37475)

Previously, layout was handling scrollable overflow and srolling area
calculation separately, only excluding the "unreachable scrollable
overflow region" at the last step. In addition, `position: absolute` was
not included in scrollable overflow calculation.

This change combines the two concepts into a single scrollable overflow
calculation and starts taking into account `position: absolute`.
Finally, `BoxFragment::scrollable_overflow_for_parent` is converted to
use early returns which reduces the amount of indentation.

Fixes #35928.
Fixes #37204.
Testing: This causes some WPT test to pass, but also two to start
failing:
- `/css/css-masking/clip-path/clip-path-fixed-scroll.html`: This seems
to fail
because script is scrolling past the boundaries of the document. This is
a
failure that was uncovered by the fixed element now being added to the
   page's scroll area.
- `/css/css-overflow/overflow-outside-padding.html`: One test has
started to fail
here because now the absolutely positioned element is included in the
scroll area,
and I think there is an issue with how we are placing RTL items with
negative margins.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/display_list/mod.rs

@@ -1100,12 +1100,7 @@ impl<'a> BuilderForBoxFragment<'a> {
 
     fn build(&mut self, builder: &mut DisplayListBuilder, section: StackingContextSection) {
         if self.is_hit_test_for_scrollable_overflow {
-            self.build_hit_test(
-                builder,
-                self.fragment
-                    .reachable_scrollable_overflow_region()
-                    .to_webrender(),
-            );
+            self.build_hit_test(builder, self.fragment.scrollable_overflow().to_webrender());
             return;
         }
 

components/layout/display_list/stacking_context.rs

@@ -1472,12 +1472,10 @@ impl BoxFragment {
             y: overflow.y.into(),
         };
 
-        let content_rect = self.reachable_scrollable_overflow_region().to_webrender();
-
         let scroll_tree_node_id = stacking_context_tree.define_scroll_frame(
             parent_scroll_node_id,
             external_id,
-            content_rect,
+            self.scrollable_overflow().to_webrender(),
             scroll_frame_rect,
             sensitivity,
         );