Undo in expression inputs disregards values being set

[shueja]

Reproduce:
Add a waypoint
Click into the waypoint X input, which will have some long decimal number like 6.321791648864746 m
Select all, delete
Type "8m", press Enter. This will not be accepted as-is, but the app will write a modified version 8 m into the input.
Click back into the input, press ctrl-Z. See the input contents as 8 m6.321791648864746 m (with all but 8 m selected)

Notes:
This does not happen if the delete button is skipped (i.e. typing directly over the selection).

Another variant of the same bug:

Reproduce:
Add a waypoint
Click into the waypoint X input, which will have some long decimal number like 6.321791648864746 m
Select all, delete
Press Enter with the input still empty. This will make JavaScript restore the original number
Click back into the input, press ctrl-Z. See the input contents as 6.321791648864746 m6.321791648864746 m with the latter copy selected.