Opening `controls_if` block mutator dislocates attached blocks

[cpcallen]

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Description

Opening the mutator in a controls_if (if/elseif/else) block will cause any blocks attached to the elseif and else inputs of the block to be temporarily dislocated.

Reproduction steps

In the Blockly playground, in v9.3.1:

1. Add an "if" block.
2. Open the block's mutator and add "elseif" and "else" clauses.
3. Close the mutator.
4. Add blocks to all the blocks inputs:
   
5. Open the mutator and observe the blocks attached to the elseif and else inputs have been dislocated:
   
6. Drag the block and observe the blocks snap back to their correct location:
   

Stack trace

Nothing in the console.

Browsers

Chrome desktop

Additional info

Discovered via this report in the Blockly forum.

[BeksOmega]

This is already fixed by #6918 ! We might want to get that merged and release a patch for this though.