Auto merge of #24736 - paulrouget:layoutissue, r=SimonSapin

layout: Ensure abs-pos elements establish block formatting contexts

Updating #16310

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13495 (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

components/layout/block.rs

@@ -1682,6 +1682,13 @@ impl BlockFlow {
     /// Determines the type of formatting context this is. See the definition of
     /// `FormattingContextType`.
     pub fn formatting_context_type(&self) -> FormattingContextType {
+        if self
+            .base
+            .flags
+            .contains(FlowFlags::IS_ABSOLUTELY_POSITIONED)
+        {
+            return FormattingContextType::Other;
+        }
         if self.is_inline_flex_item() || self.is_block_flex_item() {
             return FormattingContextType::Other;
         }