My mouse cursor appears onscreen when I look to the left or right while in-game.
I have the game set to fullscreen.
I cannot R-click while the cursor is visible otherwise I R-click my desktop and the game minimises and my computer opens the drop menu for R-clicking my desktop or a shortcut.

This is generally caused by something in the background, but a general fix is disabling visual themes in the Compatibility tab for the TESV executable.

This seems to have worked. I'll post again if it reappears.