[edit] viewport heights under 600px are not currently supported

The inner frame has `overflow: scroll` so the document can be as tall as necessary.

We vertically center the edit toolbar to keep it in place as the inner frame scrolls. This is currently using fixed positioning.

When a user sizes their screen height to under 600px, the menubar and the library are visible (probably not a bad thing), but the top part where it overlaps with the app frame is not clickable.

Possible fix: reflow the menubar to fit within the main frame, but this will cause many other challenges due to how the entire box model works around fixed positioning.

Start by reproducing issue #996 with a viewport shorter than 600px and inspect the edit toolbar, menubar, library, and inner frame layout. Trace the fixed positioning and overflow behavior described in the issue; done means the overlapped top area remains clickable while the inner frame can still scroll.

Written by the indexing model from the issue text.