Dragging window highlights last app in App Tray panel (top panel)

[SoloRobo]

Dragging window highlights last app in App Tray panel (top panel).

Open 3 apps in floating window mode.

Each app when in focus has their icon highlighted.

Dragging a window around causes the last app to be in focus to be highligted in App Tray applet, removing the App Tray icon highlight from the focused/dragged window.

Possibly illustrative screenshot:

[git-f0x]

Likely related to #453.