To reproduce.
Open page with text/image component: like /demo-project/about/subsection-articles/an-interesting-article:edit
Edit text/image component.
click the "M" in the rich text area in order to create an internal magnolia link.
> Page Chooser opens.
> Behind the dialog - the Page editor is replaced with Pages main Sub app.

close the chooser.
close the editor.
> You are in main Sub app.
Click back to editor.
> You see a blankish screen. (Sometimes, sometimes the editor looks normal.)

The editor is empty except for the toggle and the full size buttons of the actionbar.
If you resize the browser, then the editor looks OK. Page editor comes back, and the actionbar comes back.