Selection Box Get Stuck Fix

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Description

Fix the issue of when selection box get stuck when pointer leaving flowchart window in the middle of selecting blocks

What is the current behavior?

Selection box will get stuck if mouse/pointer leaving flowchart window

What is the new behavior?

Will set the selectionBox.size to 0 then repaint it.

https://user-images.githubusercontent.com/64100867/123407240-f29c1e00-d5d5-11eb-80b6-3de8be111c8a.mp4

Other information

Relates to https://github.com/snozbot/fungus/issues/1002