[WIP] Align comments with their related block (when comment sidebar is not pinned)

[adamsilverstein]

What?

Fixes #71393

This PR visually aligns comments beside (to the right of) the block they are related to. This is in the design spec for inline comments. Note that this does not apply when showing the "pinned" comment sidebar.

Why?

When commenting on a block, and replying to those comments in a thread, it makes sense to position the comments alongside the block so they are visually related. This becomes increasingly important as the number of comments or the document length increases.

How?

As discussed previously, this PR will attempt to put comments in a <Popover> component.

Plan

• When not in pinned mode, wrap comments in a Popover element
• Anchor the popover to the block itself
• Ensure popover sizes to fit comment thread
• Ensure comment popovers don't overlap
• move the visibility control from the thread to the popover so a resolved comment's container popover disappears

Challenges

• when a document has many comments they will each need some space reserved
• scrolling must remain performant even with many comments showing at once

Testing Instructions

When ready for testing:

1. Ensure comment experiment is enabled
2. leave several comments on blocks in a long test document
3. Scroll up and down in the document to verify the comments appear alongside the blocks they were left on.

Testing Instructions for Keyboard

• Same as above, scroll with keyboard

Screenshots or screencast

[adamsilverstein]

Still fixing some issues and struggling thru some challenges, here is a screencast of work in progress:

screencast.2025-09-18.15-12-15.mp4