Can tab into another ViewColumn while in single-column layout

[rezbyte]

Describe the bug
While in the single-column layout (mobile etc.), the off-screen ViewColumn can still be tabbed into.

To Reproduce
Steps to reproduce the behavior:

1. Go to the mail list while in single-column layout
2. Tab until the back button of the conversation view is focused

Expected behavior
Only the elements that are visible can be tabbed to.

Additional context
This problem was deliberately reintroduced by #7053 in order to fix to #7050. We (ivk & jat) considered the tabbing issue to be a lesser problem comparatively.