Can't select large spans of text in the GUI

The Multipass GUI does not support selecting text spanning more than one screen:

- Pulling a selection beyond the screen doesn't cause the terminal to scroll
- Scrolling while selecting causes the selection to move
- Click, ctrl+click doesn't cause text to be selected at all

> Hi; to clarify I meant in your app; if you select a part of text and want to continue above or below the current screen (so part of the previous or next screen) you can't do that.
>

_Originally posted by @alexio-dev in [#4250](https://github.com/canonical/multipass/issues/4250#issuecomment-3101839038)_

Start by reproducing text selection in the Multipass GUI, including dragging beyond the visible screen, scrolling while selecting, and click or ctrl+click. Trace the GUI terminal's selection behavior and verify that selections can extend across screens without moving unexpectedly; done means all three reported cases work consistently.

Written by the indexing model from the issue text.