To make it easier to tell which side tools are visible, the side tools for nested blocks (inside innerblocks layout areas) have a slightly different style; currently like this, with visible button borders:

This is slightly different from parent blocks, which don't display borders:

The distinction is helpful but I think can be improved — and make the side UI a little more consistent with the main toolbar — by wrapping these areas in a border instead of an always-visible border for individual styles.

Before and after:

And here's a demo:

This could extend to parent blocks as well, for greater consistency, although I didn't include that in the above demo.

Note that I also styled the borders to match the block hover styles implemented in #6773 but that could be changed as well.