I had a hell of a time closing windows even with this explanation.
If it’s still unclear to anyone else, this is what you do. You must merge the unwanted window with a different window that you want to keep. To do that, hover over the top right of a window (where the diagonal hash marks are) until you get a + symbol.
Then drag up and/or right (not down or left) to a different window, and hover over that window’s diagonal hash marks. The target window should darken a bit, with a big up or right-pointing arrow overlay showing faintly. That tells you that the merge will work. Once you have that big arrow, it’s safe to let go of the mouse and let the merge happen.
You don’t HAVE to merge with windows that are above or to the right of the unwanted window. Once you get the big arrow to appear, you can then drag down or left to merge in the other direction. Also don’t worry if merging the windows the way you want causes you to get the wrong view (like 3D view at the bottom and properties on top or whatever). Once the windows are set up correctly, you can use the up&down arrow thing to change what you’re viewing in that window.
Now I just need to figure out how to get my vertical properties page to stop drawing everything horizontal-style (so I have to scroll way to the right)