How might text be selected while typing the login password on my Mac? When I wake up my Mac I am presented with the login screen. I begin typing my password in the empty password text box. 

After typing the first few characters, the password text becomes highlighted (selected) somehow thereby causing the next character I type to delete those selected characters. 

For example, say my password is "password123"


I press power and my Mac wakes up I type "passw"
The password text becomes selected somehow
I finish typing "ord123" and hit enter
The password is incorrect since I am left with "ord123" in the password input box


Why is this and/or how can I prevent this from happening?
I know the behaviour you describe. In my case, I wake up my Mac by pressing a key and I just wait for a few secs before entering the password. The key I pressed to wake up my Mac and if I press any others in that first couple of seconds (I am often impatient to get my computer to wake up!) will get selected as you describe.

I think this is intentional behaviour by Apple, that any character typed before the password field is displayed and working is not treated as part of the password to be entered &amp; so are deleted. The behaviour though is not a good user experience. I think a better experience should be that the characters typed before the text field is ready should not be added to the text field when it is ready, thus making it clear that characters entered before the text field is ready are ignored. The adding them &amp; then selecting &amp; deleting behaviour is just odd.

My answer I suppose is this. After waking your computer, just wait a couple of secs before entering your password. I've just learnt to do that without thinking about it because it works. But your right &amp; if you have the enthusiasm report it as a bug to Apple.