Text fields persistently highlight when you press shift

The bug

I've noticed this issue for a few years and, up until now, I never knew what the issue was. I would press shift to capitalise a character then go to click and the text starts to be highlighted. I found that you have to press either control, alt, or arrow keys to stop highlighting the text field but this isn't ideal as most players wouldn't know this.

To reproduce

1. Type anything in a text field then press shift and let go
2. Move your caret by clicking
3. Text should be highlighted
4. Press either the ctrl, alt, or arrow keys to deselect

I expected to only select text when I hold shift, not when I press it once.

Code analysis

Code analysis by Malo can be found in this comment.