New dashboard no longer accommodates for heading

[kra-mo]

Talk app version: 25.0.0-alpha.1

It now looks like this on narrower screens:

A regression introduced in #18980. @nimishavijay

[kra-mo]

To be clear, this happens only at sizes between the breakpoint of the sidebar hiding, and the content clamping.

[Antreesy]

Confirmed at ~1024px width and <800px height.

Also noticed, that sidebar opens at '920/921' threshold, but 'isMobile' flips at '921/922' threshold
UPD: there's 1px mismatch between CSS and JS computation of mobile breakpoint on Chrome, if page is zoomed-out (rounding error) - upstream, but not worth fixing performance-wise