Theme preview pane overlaps page when shown

[danieltj]

I noticed that when previewing a theme in the theme directory, you'll first see a panel to the left (I guess it's a modified customiser panel?). You can collapse the panel so it's hidden from view and then show it again. At first when it's shown, it makes the page smaller so the theme scales, but when you hide it and show it again, it overlaps the theme so a portion of the page is unusable.

I think it should have a consistent behaviour by making the space for the theme smaller rather than overlapping the page. I'll attach screenshots to explain.

Example page: ​Twenty Seventeen

[danieltj]

Theme preview by default

[danieltj]

Theme panel hidden

[danieltj]

Theme panel shown again overlapping

[obenland]

In 6695:

Themes: There's no fixed adminbar sub-600px

See #3468.

[obenland]

In 6696:

Themes: Third time's a charm.

Pay tribute to different collapsing behavior between mobile and desktop views.

Props danieltj.
Fixes #3468.