Hi
I make some mis her with blender main view
how to reset the blender main screen to default ? 
there are more possibilities:
- the box at the top where you have now written ācompositingā - switch it to default
- return that manually - if you grabb viewport by zhe corner and pull it to other view port, they should mergeā¦
- delete the configuration folder (".blender" in linux, and in windows i have no idea where exactly)ā¦
āFileā - āLoad factory Settingsā would be fastest way.
If you place mouse cursor on small crosshatched triangle in upper right corner of some window and drag over to the RIGHT where SAME SIZED window is, white arrow appears and windows will be joined (one you started with will be whatās left).
Same goes when you find some triangle which could be dragged to below (same sized before) window.
If you drag triangle INSIDE window, new one will be created. So, to delete near window drag neighboring triangle over the line next to it.
If you right click on the window border and see double arrow icon, thereās a menu [Split/Join Area]. Split does just that, Join presents witeish arrow which could be pointed by moving mouse left/right up/down. That is another way of getting windows joined.
If some to join are different sizes - resize it so that itās equal with one you plan to join. Thatās important! You can not join one wide window with 2 sticking against that one. Big one needs to be split into 2 and Resized!
Go for Factory settings, and try some triangle moving. It needs a little training ;).
Plus - Docs on all this
Hope helps.