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/fragment_tree/fragment.rs

@@ -162,7 +162,7 @@ impl Fragment {
         }
     }
 
-    pub fn unclipped_scrolling_area(&self) -> PhysicalRect<Au> {
+    pub(crate) fn scrolling_area(&self) -> PhysicalRect<Au> {
         match self {
             Fragment::Box(fragment) | Fragment::Float(fragment) => {
                 let fragment = fragment.borrow();
@@ -172,17 +172,6 @@ impl Fragment {
         }
     }
 
-    pub fn scrolling_area(&self) -> PhysicalRect<Au> {
-        match self {
-            Fragment::Box(fragment) | Fragment::Float(fragment) => {
-                let fragment = fragment.borrow();
-                fragment
-                    .offset_by_containing_block(&fragment.reachable_scrollable_overflow_region())
-            },
-            _ => self.scrollable_overflow_for_parent(),
-        }
-    }
-
     pub(crate) fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
         match self {
             Fragment::Box(fragment) | Fragment::Float(fragment) => {