Auto merge of #26049 - mrobinson:hoisting-icb, r=SimonSapin

layout_2020: Stop throwing away hoisted boxes in nested absolutes

We were previously throwing away some boxes hoisted to containing blocks
for all descendants when they were contained by absolutes. This prevents
panics in existing web platform tests that would otherwise be triggered
by the addition of the `unreachable!` statement.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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components/layout_2020/display_list/stacking_context.rs

@@ -736,10 +736,7 @@ impl AbsoluteOrFixedPositionedFragment {
         let hoisted_fragment = self.hoisted_fragment.borrow();
         let fragment_ref = match hoisted_fragment.as_ref() {
             Some(fragment_ref) => fragment_ref,
-            None => {
-                warn!("Found hoisted box with missing fragment.");
-                return;
-            },
+            None => unreachable!("Found hoisted box with missing fragment."),
         };
 
         let containing_block = match self.position {

components/layout_2020/lib.rs

@@ -6,8 +6,6 @@
 #![feature(arbitrary_self_types)]
 #![feature(exact_size_is_empty)]
 
-#[macro_use]
-extern crate log;
 #[macro_use]
 extern crate serde;
 

components/layout_2020/positioned.rs

@@ -226,8 +226,8 @@ impl PositioningContext {
 
         let mut new_fragment = fragment_layout_fn(&mut new_context);
         new_context.layout_collected_children(layout_context, &mut new_fragment);
-        *for_nearest_containing_block_for_all_descendants =
-            new_context.for_nearest_containing_block_for_all_descendants;
+        for_nearest_containing_block_for_all_descendants
+            .extend(new_context.for_nearest_containing_block_for_all_descendants);
         new_fragment
     }