Editor toolbar disappears when editing large page

Issue Summary

When attempting to edit a large page having scrolled enough for the top navigation bar to disappear, the editor will be missing a text editor toolbar.
Issue reproducible on 6.12.2 and 6.13.4

Steps to Reproduce

1. Create a long, content-rich page, long enough to get a scroll bar
2. Scroll the page enough for the top navigation bar to disappear
3. Click "e" to enter page editor

Expected Results

• Editor loads with a toolbar containing text editing tools.

Actual Results

• Toolbar will be missing

Workaround

Not reproducible on 6.13.12, 6.15.9 or 7.1.1
Exiting editor, scrolling to the top of the page and clicking edit again will bring back the toolbar.
Reloading the page should bring back the editor toolbar