While fiddling with the layout in Android Studio I somehow turned my logcat window into a floating window. I cannot for the life of me restore it to docked mode.
Seems the OP is running into this issue on some flavor on Linux. For people with OS X, see this answer.
If you select the little cog icon at the top right of the floating window, and deselect floating mode, it should pop back to pinned mode:
If you don't see the little cog, you can also make sure that the floating logcat window is selected, and then go to Window-> Active Tool Window-> Floating Mode