Dragging blocks stops their selection highlight

[BenHenning]

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Description

Attempting to drag a block results in the block's selection highlight disappearing until dragging finishes (at which point it's restored).

Reproduction steps

1. Open the playground for v12.
2. Insert any block.
3. Click to drag the block around.
4. Observe that selection highlight is missing during dragging.

Stack trace

Screenshots

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[BenHenning]

This is almost certainly caused by focus being lost during drag and the auto-selection mechanism in BlockSvg resulting in selection being dropped.

I'm uncertain why focus is lost--that requires more digging.

[BenHenning]

NB: This is also broken for workspace comments (and technically bubbles, though they don't have an actual highlight to show).

[BenHenning]

Fixed in #9034.