Improve consistency of Zoom Out inserters and dropzones

[getdave]

As @talldan notes below, the Zoom Out mode dropzones in the canvas can be a little inconsistent as to when they show hide.

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There's one small thing that I noticed that could still be improved. If you cancel the drag by dragging back to the inserter and releasing, the blue inserter button reappears and the gray insertion zone doesn't hide until hovering over the canvas:

Ah yes. I think I know why. I will try and fix that in a followup as it's likely to be fairly complex.

Originally posted by @getdave in #65789 (comment)

[getdave]

Closing in favour of #65927