[v13.0.0-beta.0] Move mode icon position issues

[microbit-robert]

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Description

#9699 addresses the more fundamental issue with inserting a block from the flyout via keyboard. However, the move mode icon is initially positioned on the top-left of the block. On the first arrow key press (even if this doesn't move the block, i.e., no constrained positions) moves the icon to the top-right of the block.

For blocks already on the workspace, the move icon's horizontal position is dependent on the widest block in the stack that is being moved which can leave it detached from the block that the move was triggered on. See screen shot below.

Reproduction steps

1. Check out the Blockly v13 tag
2. npm start from the relevant dir
3. Insert a block from the flyout via keyboard and observe move mode icon position
4. Arrow key press
5. Observe move mode icon position

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[gonfunko]

@microbit-robert What do you think about moving the indicator to the top left (top right in RTL) corner? I have a fix for the initial positioning, but for historical reasons blocks return their bounds including their children, so adjusting that behavior would be a breaking change and customizing it would require otherwise-generic code special-casing blocks (which also introduces an import cycle). AFAIK the top left corner is contentful for all draggable things, and I don't recall if there was a specific reason for the top-right placement or if it was just arbitrary. I can make it work, but if there's no strong preference to prefer the top right corner just using the top left would be cleaner.

[microbit-matt-hillsdon]

I'm leaning towards keeping it as it was for two reasons: (1) onboarding resources (videos etc) created last year based on the previous Blockly+plugin behaviour, and (2) a slightly better feel to the initial experience of move mode which is quite likely to be with a top-left block.