Block selection indicator partially obscured in Zelos renderer

[cpcallen]

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Description

There are certain circumstances when the selection highlight ends up being partially obscured by other blocks when using the Zelos renderer.
E.g.:

• When dragging a new block from the toolbox into the middle of a stack.
• When pasting a block into the middle of a stack. (Requires keyboard nav.)

Originally reported by @microbit-robert in RaspberryPiFoundation/blockly-keyboard-experimentation#252 (comment).

Reproduction steps

1. In the advanced playground, set render to Zelos.
2. Create a stack of any two statement blocks.
3. Drag a third block between the first two
4. Observe the selection highlight on the block you have just placed.

Screenshots

Appearance at step 4:

Appearance at step 6: after selecting a different block and then the just-placed one—arguably the expected appearance at step 4:

Browsers

Chrome desktop

[cpcallen]

Tentatively assigning this to @gonfunko; probably not a top priority as I suspect this behaviour has existed for a while and is only a cosmetic problem, but might be nice to fixing soon if there's a good+easy solution since it was noted by one of our partners while working on the keyboard nav plugin.