What?

Follow up on #71308 to make blocks associated with comments more clear.

Why?

#71308 dispatches flashBlock action when the related comment is selected. Because flashBlock only highlights the block for a split second, it can be difficult to see the block associated with a comment, and you have to click the comment again to flash the block.

How?

I realized we might be able to reuse the logic for associating list views with blocks. So in this PR, I'm introducing the following behaviors:

• Highlight the block when a thread is focused or moused over.
• Unhighlight the block when a thread loses focus or is moused out.
• Apply useDebounce hook to toggleHighLight to reduce the nastiness of the block when you move the mouse quickly.

Testing Instructions

• Add comments on blocks.
• Mouse over or mouse out on a thread and confirm that the releated block is highlighted.

Testing Instructions for Keyboard

• Use the tab key to focus on an element within a thread.
• When focus moves into a thread, the block is highlighted.

Screenshots or screencast