There is a float issue in eZ Publish / webin / flow, that is causing Online Editor 5.0 to not use full width.

This happens where there is floating (left/right) embed tags at the end of the content your translating.

Suggestion 1:
Add "clear: both;" to div.oe-window in core.css, and make sure this style is defined in all core.css files(currently only defined in site.css for admin design).

Suggestion 2:
(optionally also with suggestion 1) fix the float issue on the translation div block, so it breaks the float elements inside it (float-break in webin).