docs: fast visibility now supports Shadow DOM with documented caveats

[mschile]

Pairs with cypress-io/cypress#33737, which makes the experimentalFastVisibility algorithm shadow-DOM aware (resolves cypress-io/cypress#33046).

This PR is stacked on top of #6434 because it builds on that branch's clipping-ancestor docs.

What changed

• Replaced the "Shadow DOM is not yet supported" limitation in two places (experiments.mdx and interacting-with-elements.mdx) with the new state: Shadow DOM is now supported. The remaining divergences from the legacy algorithm are documented as three concrete buckets, all rooted in fast's reliance on elementFromPoint:
  • Cover narrower than the underneath element — fast samples four corners and the center, while legacy samples only the center, so a subject with an uncovered corner can be reported visible by fast and hidden by legacy.
  • pointer-events: none on the subject — browsers skip such elements in elementFromPoint, so no sample point can land on them. Setting pointer-events: auto on the subject is the workaround.
  • Clipping ancestor between the subject and its containing block — legacy uses offsetParent rules to ignore that ancestor; fast does not.
• Refined the existing clipping-ancestor paragraph from docs: fast visibility scrolls off-screen elements into view #6434 to (1) cover all four clipping overflow values (hidden, clip, scroll, auto), (2) note that in-bounds subjects merely below the fold are scrolled into view normally, and (3) state that the check now walks shadow root boundaries.

Steps to test

Render the docs locally and visit:

• docs/app/references/experiments#Experimental-Fast-Visibility — confirm the caution block now points at the Limitations section.
• docs/app/core-concepts/interacting-with-elements#Experimental-Fast-Visibility — confirm the Limitations bullets describe the three caveats and the clipping-ancestor paragraph reads correctly.

---

Note

Low Risk
Low risk: documentation-only updates that clarify behavior and known edge cases of experimentalFastVisibility, with no runtime or API changes.

Overview
Docs update for experimentalFastVisibility: Shadow DOM is now documented as supported, replacing the previous “not supported” limitation and adding concrete caveats tied to elementFromPoint (corner-only visibility, pointer-events: none, and clipping-ancestor semantics).

Clarifies scrolling/clipping behavior for overflow: hidden|clip|scroll|auto, including that in-bounds-but-offscreen subjects still scroll into view and that clipping checks traverse shadow roots, and updates the experiments page to link readers to the detailed limitations section.

^{Reviewed by Cursor Bugbot for commit 2aa82d6. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cypress]

cypress-documentation    Run #1075

Run Properties:   Passed #1075  •  2aa82d646a: docs: fast visibility now supports Shadow DOM with documented caveats

Project	cypress-documentation
Branch Review	mschile/pensive-black-034995
Run status	Passed #1075
Run duration	05m 39s
Commit	2aa82d646a: docs: fast visibility now supports Shadow DOM with documented caveats
Committer	Matthew Schile
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	322
View all changes introduced in this branch ↗︎

UI Coverage  47.51%
Untested elements	337
Tested elements	3

Accessibility  96.88%
Failed rules	1 critical   3 serious   4 moderate   1 minor
Failed elements	35