layout: Add effective_visual_size to PaintTimingHandler - #43378

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Align size calculations as much as possible close to W3C specs

Testing: More WPT Passed
Fixes: Part of servo#42000

---------

Signed-off-by: Shubham Gupta <shubham.gupta@chromium.org>
Signed-off-by: Shubham Gupta <shubhamg13.work@gmail.com>

Hi, I'm a spec champion for the Container Timing feature and I wanted to
add it to Servo (behind a flag).

The Container Timing API enables monitoring when annotated sections of
the DOM are displayed on screen and have finished their initial paint. I
am one of the spec authors, and i've been helping this land in both
[Chromium](https://developer.chrome.com/blog/container-timing-origin-trial)
and [Firefox](https://phabricator.services.mozilla.com/D279373)

This PR implements the [Container Timing
API](https://github.com/WICG/container-timing), following the same
layout > paint > constellation pipeline already used for LCP.

  Steps for calculating Container Timing:

1. When an element has the `containertiming` attribute, a [flag is
propagated down through its
descendants](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/script/dom/node/node.rs#L378-L387)
(set on the Node/fragment) so layout can cheaply tell "is this inside a
container-timing root" without walking back up the tree on every
fragment.
2. While building the display list, each painted text/image fragment
that belongs to a container-timing root [reports its viewport-clipped
size and rect to a
PaintTimingHandler](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/layout/display_list/mod.rs#L706-L718),
similar to how LCP candidates are collected.
3. PaintTimingHandler keeps one running record per container root (keyed
by a stable node ID, since the containertiming value itself is optional
and not unique), accumulating size/rect across paints, and tracks which
containers were actually touched since the last hand-off.
4. Layout [hands the finished per-container records to
script](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/layout/layout_impl.rs#L1561-L1567)
as part of the reflow result
5. once webrender confirms that epoch composited, [it sends
PaintMetricEvent::ContainerTiming(paint_time,
ids)](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/paint/painter.rs#L585-L589)
to script, which stamps the already-stored records with the real
paint/first-render time and queues the finished
PerformanceContainerTiming entries.

Testing:
- There are plenty of [Web Platform
Tests](https://wpt.fyi/results/container-timing/tentative?label=master&label=experimental&aligned&q=container%20timing)
for Container Timing. I have been testing the build against these as i
have been going along, 19/29 tests pass.
- There are tests for some components, such as paintedRegion

Follow ups & investigation:
- `lastPaintedElement` selection picks by traversal order, not largest
area
- [Tidy Up] The `effective_visual_size` check was [originally written
for LCP](servo#43378), but is useful for
Container Timing as well, we should make this API generic

---------

Signed-off-by: jasonwilliams <jase.williams@gmail.com>

Hi, I'm a spec champion for the Container Timing feature and I wanted to
add it to Servo (behind a flag).

The Container Timing API enables monitoring when annotated sections of
the DOM are displayed on screen and have finished their initial paint. I
am one of the spec authors, and i've been helping this land in both
[Chromium](https://developer.chrome.com/blog/container-timing-origin-trial)
and [Firefox](https://phabricator.services.mozilla.com/D279373)

This PR implements the [Container Timing
API](https://github.com/WICG/container-timing), following the same
layout > paint > constellation pipeline already used for LCP.

  Steps for calculating Container Timing:

1. When an element has the `containertiming` attribute, a [flag is
propagated down through its
descendants](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/script/dom/node/node.rs#L378-L387)
(set on the Node/fragment) so layout can cheaply tell "is this inside a
container-timing root" without walking back up the tree on every
fragment.
2. While building the display list, each painted text/image fragment
that belongs to a container-timing root [reports its viewport-clipped
size and rect to a
PaintTimingHandler](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/layout/display_list/mod.rs#L706-L718),
similar to how LCP candidates are collected.
3. PaintTimingHandler keeps one running record per container root (keyed
by a stable node ID, since the containertiming value itself is optional
and not unique), accumulating size/rect across paints, and tracks which
containers were actually touched since the last hand-off.
4. Layout [hands the finished per-container records to
script](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/layout/layout_impl.rs#L1561-L1567)
as part of the reflow result
5. once webrender confirms that epoch composited, [it sends
PaintMetricEvent::ContainerTiming(paint_time,
ids)](https://github.com/jasonwilliams/servo/blob/403908fa99f47da3e18faaca88d2abdb01498cf7/components/paint/painter.rs#L585-L589)
to script, which stamps the already-stored records with the real
paint/first-render time and queues the finished
PerformanceContainerTiming entries.

Testing:
- There are plenty of [Web Platform
Tests](https://wpt.fyi/results/container-timing/tentative?label=master&label=experimental&aligned&q=container%20timing)
for Container Timing. I have been testing the build against these as i
have been going along, 19/29 tests pass.
- There are tests for some components, such as paintedRegion

Follow ups & investigation:
- `lastPaintedElement` selection picks by traversal order, not largest
area
- [Tidy Up] The `effective_visual_size` check was [originally written
for LCP](servo#43378), but is useful for
Container Timing as well, we should make this API generic

---------

Signed-off-by: jasonwilliams <jase.williams@gmail.com>