https://github.com/user-attachments/assets/9aba75ff-4190-4a85-89ed-d3f3aa53d3b0
Among other things this adds a new `EmbedderMsg::ShowSelectElementMenu`
to tell the embedder to display a select popup at the given location.
This is a draft because some small style adjustments need to be made:
* the select element should always have the width of the largest option
* the border should be part of the shadow tree
Apart from that, it's mostly ready for review.
<details><summary>HTML for demo video</summary>
```html
<html>
<body>
<select id="c" name="choice">
<option value="first">First Value</option>
<option value="second">Second Value</option>
<option value="third">Third Value</option>
</select>
</body>
</html>
```
</details>
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---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
* Sort stacking contexts and stacking containers by painting order
* fix stealing of stacking containers; fix interleaving with fragments
* actually positioned stacking containers should be stolen too
* update expectations and clean up panic changes
* rework naming and docs
* rename s_c_a_p_s_c to real_s_c_a_p_s_c; fix docs
* rename InlineStackingContainer to AtomicInlineStackingContainer
* rework debug logging to use PrintTree
* clean up docs and PrintTree output
* don't panic unless cfg!(debug_assertions) is true
* update expectations
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
