I have two external monitors hooked up to a laptop and, oddly enough, when I drag a window onto the laptop screen, the window vanishes.

• When I minimize a window that has disappeared, it appears to minimize from below the visible screen area
• If I drag it *partially* on the screen, that's fine
• If I drag it to the top of the screen (so it's maximized) then click the windowed mode button, that's fine too
  
• Same for edges
• If I drag it after doing any of this, it disappears unless I drag it partially onto another window
• If I try to resize the window, so that it shrinks until it's only on the laptop screen, the window will stop shrinking to stay on the external monitor
• Alt-dragging can bring the window onto the external monitor, but then I can't change the window width (regular dragging still makes it disappear)
• Persists between reboots

Anybody know how I can keep windows on the screen? I'm using ubuntu 16.04 and plasmashell 5.8.7