[GREENFOOT-541] Empty slots do not expand when the frame cursor is moved into them

Most frames expand by the size of the cursor when the cursor enters them (and reduce accordingly when the cursor leaves). Empty frames are an exception to this - they remain the same size when the frame cursor enters/leaves.

This has the perturbing effect of causing the document size to "wobble" as you use the up/arrow keys to navigate. When entering an empty frame, the entire document becomes slightly shorter, and when leaving the empty frame the document becomes slightly longer.

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Issue metadata

• Issue type: Bug
• Priority: Medium
• Fix versions: 3.7.0