Message input field is not scrollable if there's more text than what max height can show

[Porkepix]

In case the message input field contains more text than what the max height and the current length allows for, we can't scroll the typed text. As the "Begin" and "End" keys only send to the beginning and start of the current line and not of the whole message and as the arrow keys move between previous and last messages, it only let the left/right key to users to (slowly) move higher up.

Note that the arrow keys behavior also means we can't easily shift + bottom/up arrows to select quickly text over several lines.

[deepspaceaxolotl]

I opened #1604 regarding the up and down arrow key behaviour in general! The shift to select use case is also good to bring up, yes.

I'd like to see a scrollbar scrollable with either up and down arrow keys (once they can be used on the current message) and a mouse appear for long messages.