Found the "issue" I think
So, I have been having this issue and here is what I think is happening.
If you open a text and do NOT have the text field highlighted, if you happen to start a message with "t" which, being one of the MOST common used letters and being THE most common first letter, at least in the English language. the 't' button goes to the "top" of the text. If you have a substantial text history with someone, this will take a LONG time. and while it does it, it prevents you from sending a text for some reason.
Shortcut keys:
Shortcuts: BlackBerry Hub - How To - BlackBerry Q10 Smartphone - 10.1
Here are steps to reproduce:
1) Open a text (preferably with someone whom you text frequently)
2) click on a previous text (this will remove focus from the input, note that you should see Enter a message but no blinking cursor)
3) Press "T" ( I am not sure which other 'shortcut' keys may do this too)
4) click on the text entry field (note blinking cursor)
5) Try to type a message or send
I have seen in step 5 that sometimes you can't type a message, nor can you Click the ... and Send Message (greyed out).
Options:
1) Wait, an indefinite amount of time for the process to stop going to the "top"
2) reboot the phone (note you shouldn't be doing battery pulls, all you need to do is hold the power button and do a reset.