Auto merge of #18188 - mrobinson:overflow-stacking-context, r=mbrubeck

Allow overflow:scroll without a stacking context

Fix the long-standing bug where items that are positioned and have
overflow:scroll or overflow:auto automatically create stacking
contexts. In order to do this we need to fix another bug where display
list sorting can put a Clip or ScrollFrame definition after the first
time it is used in a display list.

<!-- Please describe your changes on the following line: -->

---
<!-- 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
- [ ] These changes fix #__ (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. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18188)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions