The block toolbar is both sticky, and scrolls with until the selected block is out of view, but it can also — when near the top of the screen, flip below the block. So two behaviors, sticky and below.

When you use the mover control on a block that's partially scrolled out of view, what appears to be a conflict between those two behaviors makes the toolbar shift places, seemingly at random, as shown in this GIF:

As shown are the steps to reproduce:

• Edit a post or page with a lot of blocks.
• Select the first block, and scroll down so it's partially obscured and the block toolbar has moved down to overlay the block.
• User the mover control to move the block downwards and upwards, and observe how it shifts position.

Expected result: the block toolbar should always stay fixed in place.