Deadzone in `NumericUpDown` control

### Bug explanation

The `NumericUpDown` control has a dead zone where, when clicking, it does not focus the element.
I assume this has to do with the internal `ScrollViewer` in the `TextBox`

### Version

5.3.0

Start by reproducing the dead zone in the NumericUpDown control on version 5.3.0, then inspect the control and its TextBox's internal ScrollViewer to identify where clicks fail to focus the element. Done means clicking throughout the control consistently focuses it without breaking existing NumericUpDown behavior; add or update a focused regression test if the relevant test location is found.

Written by the indexing model from the issue text.