[BLUEJ-866] Empty import canvas doesn't fold right in Stride editor for Java preview

If you have an empty imports canvas in the Stride editor, then when you exit Java preview (entering is fine), it briefly animates to nothing, then reappears again, making all the source code (all of which is below the imports) bump up and back down again. Probably wasn't noticed before because all Greenfoot Stride classes usually have the greenfoot.* import, so this mainly comes up in BlueJ.

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Issue metadata

• Issue type: Bug
• Priority: Low