When accessing the command block GUI and typing in the command, if you press shift to make a capital letter or secondary character (e.g. ?, >, etc.), then let go of shift, if after that you click elsewhere in the text, it will highlight the text. This makes it extremely annoying when trying to type in commands, as the shift key is commonly used, and navigating to other parts of the text is also a common action. To be clear, this is not when holding shift and then clicking elsewhere in the text, it happens after shift has been released, meaning that the shift key is activating the text selection, but doesn't deactivate it when the key is released. Hitting ctrl or the arrow keys seems to exit out of that mode and things are back to normal until shift is used again.