Before this, there were two weird behaviors when using the keypad
buttons to add numbers into the input field:

- On each click, the input is visually blurred then focused again, which
  creates a flickering effect of the bottom border.

- When the input is full and numbers keep being added, between mousedown
  and mouseup, the input is scrolled back to the beginning and then
  scrolled back to the end where the number is added (at least if using
  Chrome). This seems to be the case at least since [1], even though the
  input was quite different at the time.

Both behaviors are first caused by the same thing: the input being
blurred on click. This is now prevented by preventing the default
behavior of the buttons pointerdown.

After that, the second problem was not entirely fixed: now the scroll
of the input never changed, meaning that numbers added "at the end" were
not visible as the scroll stayed in place. The cursor is now forced back
into view by blurring and focusing again immediately. This simulates
nearly the same behavior as using a real keyboard. The only (found)
difference is that if the cursor is in the middle when adding a number,
and the scroll is not at the start, the cursor stays visually in
position as more characters are added. This is judged acceptable (also
this is only the case on Chrome).

[1]: https://github.com/odoo/enterprise/commit/52b3065993c41c6b7c65dda586a66fdd865b3afd

This PR also reviews the longpress implementation and fix a Firefox glitch.

task-5366875