Fix invisible drop target indicator caused by is-drop-target class being applied to dragged block

[talldan]

Description

Since #23024 the block being dragged is set to display: none.

I noticed a small issue where if that block is the one considered a drop target, it would have the is-drop-target class name applied, but because it's set to display: none, the drop target would not be visible.

To users, it'd seem like they were dragging a block into a valid position, but the blue drop target indicator wouldn't appear.

This PR fixes things by making sure the block being dragged is never considered a drop target.

In the future, I think it'd be great to not use the block border itself as the thing that indicates a drop zone, but in the short term this seems like the best way to fix the issue.

How has this been tested?

1. Bigger blocks are easier to test with, so add three image blocks. No need to upload an image, just keep the placeholder visible.
2. Drag the middle image block, and position the cursor so that it's on the lower half of the first block
3. Observe that on this branch the blue drop indicator displays, while on master it's not visible.

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: +37 B (0%)

Total Size: 1.13 MB

Filename	Size	Change
build/block-editor/index.js	109 kB	+37 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
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[jasmussen]

Thank you!

This just needs a code review, looks like!

[ellatrix]

In the future, I think it'd be great to not use the block border itself as the thing that indicates a drop zone, but in the short term this seems like the best way to fix the issue.

Yes, this we should do this differently :)