Could someone with a version of Altium prior to 21.8.1 just check something for me, please. On the Windows menu, if you select "Arrangle all windows vertically" (or horizontally), what happens? Here on 21.8 it maximises the Altium window whereas I would expect it to tile the document windows within the parent, not affecting the parent size.
The right click on a document tab and "Split vertical", or horizontal, do what they say, but obviously only affects that window.