Editable area becomes very small when editing maximized in-place Source mode with Realtime active

Steps to reproduce

1. Go to a page (e.g. Sandbox.WebHome)
2. Click edit to edit the page in-place
3. Click 'Maximize' button from the toolbar to maximize the editing area
4. Click 'Source' button
5. Observe the editable area

Expected results

The editable area becomes maximized (covering the entire available space).

Actual results

The editable area becomes very small, covering only a small part of the top space.

The issue seems to reproduce only while editing in in-place mode with Realtime active and it reproduces also on XWiki 16.10.8 (Realtime active).

It can be considered a regression from the users point of view.