There is some interesting history to this change:

• Crash when attempting to proceed through Cloudflare Verification #33495: Servo crashes on
  Cloudflare's turnstile, because we didn't compute style for
  elements inside shadow trees
• Include non-shadowdom children of shadow hosts in style calculation #34298: Resolves the issue
  by computing style for children of a potential shadow root,
  in addition to the children of an element
• layout: Lay out Shadow DOM elements #34701: Changes layout
  of elements with shadow roots such that only the contents
  of the shadow root are laid out

Now, we compute style for both the children of the element and a potential shadow root, but only lay out the contents of the shadow tree (if there is one).

This behaviour is not technically incorrect, since regular children are not included in layout their style doesn't matter. However, it is inefficient: the only case where we need to compute style for a child of a shadow host is when that child is an assigned slottable in a slot somewhere else.

This part 1/n of upstreaming the changes necessary to lay out <slot> contents. Note that trying to compute style for contents and children of shadow hosts will crash in stylo, since it expects to see eachelement only once.

try run

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes are a part of Support ShadowDom #34318
• There are tests for these changes (covered by wpt)