Window list elements gets stuck under cursor when hovered

Sometimes when dragging windows in the windows list they get stuck in a state where they will be grabbed/dragged as soon as they are hovered in the window list (without any mouse button being pressed). The issue does not disappear until the window is closed or cinnamon is restarted.

Steps to reproduce:

1. Open terminal emulator
2. Open some other windows (like nemo)
3. Minimize terminal window
4. Hover the terminal window in the window list until the preview appears
5. Click and drag the window list element upwards so that the mouse cursor leaves the window list before the element is grabbed
6. At this point on of two things usually happen:
   • The terminal list element gets grabbed as soon as the mouse enters enters the window list (this can also be seen as a bug but is not a big problem since it goes away after you drop the element)
   • The terminal list element gets grabbed as soon as it is hovered in the window list, dropping the element with a mouse click or escape will not make the element return to normal, it will be grabbed again when hovered.

Installation:
Linux Mint Cinnamon 17.3 (upgraded from 17.1->17.2->17.3) running in VMware workstation 10
Cinnamon 2.8.6
Settings should be default except that i may have increased drag and drop threshold (can't find it in cinnamon-settings now) due to suffering from problems described in #1748/#1857/#3280.

Start by reproducing the problem in Cinnamon's window list using the listed terminal, minimize, hover, and drag steps. Trace the window-list hover and drag handling around the point where the cursor leaves the list; done means an element never becomes draggable without a mouse button and returns to normal after dropping or pressing Escape.