I’ve been experiencing this bug on Windows & Chrome for some time but kept forgetting to report it. When editing text in a rich text field, if I press the “Page Up” or “Page Down” button on the keyboard, the entire layout seems to shift to the left and a blank area open up on the right hand side of the panel. The only way to fix this is to refresh the page or close the entity and re-open it.
This also occurs in the fibery desktop app.