How do you close windows/tabs in 2.8? I've tried a lot of stuff, but always end up opening up more.
Where you click exactly and the direction you move the cursor determines what pane is affected and whether the operation is a split or a join. (That's why you would accidentally create more panes when you mean to close some).
See the following images. The red dot shows where the cursor is clicked, and the green arrow shows the direction of cursor motion. (To ensure you're clicking on the correct side, click within the panes as shown by the red dot instead of the tiny black corner gap).
Join from left to right (keeps left pane; removes right pane):
Join right pane to left (keeps right pane; removes left pane):
Split left pane into 2 pieces:
Split right pane into 2 pieces: