Consider making the canvas selection further right to the overview ruler/scroll bar

[Tyriar]

After #5096 the scroll bar is presented consistently across all browsers. It would look more polished if we extended selections that wrap across lines up to the scroll bar.

So instead of this:

Do this:

Doing so should make resizing horizontally feel smoother:

This will involve extending the size of the canvas a little.

[Tyriar]

Note that this also makes sense as it's like how editors often show that the trailing \n is selected: