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#1
Full screen is as the name suggest, full screen.
The other case isn't side by side, it's a resizable screen. You can go to a border and resize it an move it around the page.
It seems full screen as it is almost the size of a full screen.
Try to make it smaller, hit the full screen and then resized. You'll see it has returned to the previous size.
'Side by Side' or 'Stacked' works only on the windows that are not minimized to the taskbar. The windows must be either maximized or resized and should be on the desktop.
Since minimized windows are ignored, I think you have only one window maximized and all others minimized to the taskbar.
Try clicking on the windows that you want to show 'Side by Side' or 'Stacked' (ie. open them to the desktop) and then select the option.