Mover control: Block toolbar no longer sticks to block that's being moved until you scroll

[jasmussen]

Steps to reproduce:

• Insert several paragraphs
• Select a paragraph and click the mover control to move it

Observe that the block toolbar scrolls out of view and stays in place, until you scroll the page.

[talldan]

I can reproduce it. It seems to work ok if there's not enough content to result in a scrollbar, though it does look a little glitchy at times:

Kapture.2022-09-19.at.16.48.24.mp4

But when I try the Gutenberg demo post, it's easily reproducible (it looks the same as the screen cap in the description, so I won't add another).

I think this should be fixed for 6.1, so I'm adding it to the board.