follow mouse focus does not update after tab change

[Radu]

Repro instructions:
 - create two tabs
 - split the first tab in two vertically and position the mouse on the left split
 - switch to another tab (using the keyboard), and move the mouse to the right
 - switch back to the first tab; now the mouse is over the right split but the left split still has focus; you have to move the mouse to the left and back to the right to fix the focus.

I see this with both the old experimental gtk3 version as well as with 1.91. It happens with the default config.

Terminator is running on Ubuntu 16.04.1 but with a remote VcXsrv X Server (version 1.19.3.4).