I've noticed that the form items are quite small on mobile device.
In Seven we increased the width on narrow devices:
#1751150: Improve usability of forms on touch screen and small screen devices
I think we should do the same here, it's a nice usability win for touch devices.
Screenshot showing vanilla 9.0.1 on mobile with the user login block added to the sidebar first region:
Remaining tasks
Write a patch that changes all form items similar to #1751150: Improve usability of forms on touch screen and small screen devices
Review