Container Timing emits no entry for a display:contents container

Run it

chrome --enable-blink-features=ContainerTiming https://static.januschka.com/i-568304686/

What happens

Both containers carry containertiming and hold one paragraph of text. The only difference is the container's own display value.

<div containertiming="contents-container" style="display: contents">
  <p>text inside the display:contents container</p>
</div>

<div containertiming="block-container" style="display: block">
  <p>text inside the display:block container</p>
</div>

Root cause

Attribution moved from a DOM node flag to the pre-paint tree walk in r1684899 (CL 8098545, which removed the legacy implementation). ContainerTimingPaintAttributionTracker::UpdateOnPrePaint() is called once per LayoutObject, and a display: contents element generates none. Its containertiming and containertimingignore attributes were therefore never read, so painted descendants resolved to the next laid-out root - or to nothing.

The legacy implementation walked parentElement() at paint time, which steps through boxless elements, which is why the entry was emitted before the switch.

Fix

While visiting a layout object, the tracker now also applies the container timing attributes of the boxless DOM ancestors between that object's node and its nearest laid-out ancestor, outermost first, so nested roots still chain. Adding or removing the attribute on a display: contents element dirties the nearest laid-out ancestor instead of its own (absent) layout object.

third_party/blink/renderer/core/paint/timing/
    container_timing_paint_attribution_tracker.cc
  UpdateUnlaidOutAncestorRoots()  // new
  UpdateContainerRoot()           // extracted, shared with the walk
third_party/blink/renderer/core/paint/pre_paint_tree_walk.cc
  PrePaintTreeWalk::UpdateContainerTimingContext()
third_party/blink/renderer/core/html/html_element.cc
  HTMLElement::MarkContainerTimingChangedForPrePaint()

Web platform tests: external/wpt/container-timing/tentative/containertiming-display-contents-with-child-img.html and containertiming-with-display-contents-child-ignore-and-child-img.html.

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