[OpaqueRange] Supports CSS anchor positioning

[yisibl]

When the page scrolls, users typically expect the popover to remain at the cursor's current position.

[yisibl]

cc @stephanieyzhang

[stephanieyzhang]

Thanks for trying out OpaqueRange and for filing this!

When proposing OpaqueRange, CSS Anchor Positioning support was something we considered as future work, so I’m glad to see there is genuine interest in it. This is not planned for the initial version of OpaqueRange, but it is something we may explore in the future.

One question while I have you: OpaqueRange currently inherits the read-only startOffset and endOffset properties from AbstractRange, but as part of user research we're considering whether to keep them or remove for OpaqueRange. Have you used them in your integration, or would you find them useful? Any feedback there, or other improvements you'd like to see in OpaqueRange, would be really helpful!