If a new application is opened while dragging an icon, unity hangs

Update: Unity 6.4 is revision 2632, so this is actually fix committed for Unity 6.6.

This bug was fixed in the package unity - 6.6.0-0ubuntu1

---------------
unity (6.6.0-0ubuntu1) quantal-proposed; urgency=low

  * New upstream release.
    - Fixes non-escaped character sequences in dash previews (LP: #1039020)
    - Updated background layer for preview cover-art and details panels
      to be 10% low-light
    - Expand a PlacesGroup if it is the only category that contains results
      (LP: #950710)
    - Update unity autopilot tests to match autopilot API
    - Updated the convert files to fix some typos in the key names
    - Add gmodule dependency
    - Activate proper result if the categories aren't displayed in-order
      (LP: #1040101)
    - Refactor device launcher icons (LP: #713423)
    - LauncherController: make the controller enable the launcher struts,
      based on hide-mode option (LP: #1044005)
    - Launcher: make always possible to drag an icon to the bottom or top
      of its sub list (LP: #1043968)
    - Don't desat bfb/hud icon in DNDReset (LP: #1043963)
    - Progressively adjust the speed of the minimize animation. First
      time it is used is slower, then speeds up the more it is used.
      (LP: #1017510)
    - Implement new ordering of categories for home lens. (LP: #1043915)
    - UnityWindow now implements ScaleWindowInterface (LP: #876017)
    - Launcher: restore an icon position after that the dragging has been
      cancelled (LP: #955561)
    - LauncherDragWindow: cancel drag on window mapped/unmapped
      (LP: #1044723)
    - Now there is a check of an override color in RefreshColor (which is
      called when a PropertyNotify event happens). Also added a check in
      FullySaturateColor to a void division by zero. (LP: #975350)
    - Queue redraw after cover-art texture is updated from a url/file source.
      (LP: #1043947)
    - Fixed ability to delete glib::Source wrapper during its callback
      (LP: #1044823)
    - Close preview when dash is hidden. (LP: #1045298)
    - LauncherModel: rewrite the Reordering functions to keep the icon
      priority deltas (LP: #761155)
    - Make sure we can pass extra hints when activating preview actions.
      (LP: #1046352)
    - UnityWindow: scale window code improved (LP: #1033935)
    - The mouse will now cause the HUD buttons to change selection
      (LP: #1042692)
    - "Alt+Space" shortcut to reveal the window menu is not hardcoded, but a
       Compiz key option. " (Hold)" should also be translated. Made all
       Compiz plug-in names and all Compiz plug-in option names in
       unityshell.cpp static constants.
    - remove unity --reset, it's not anymore really needed now that we are
      in stable days of unity and we moved to gsettings
    - Removed the variables 'oldPrev' and 'oldNext' which got assigned the
      value NULL, but then were never used
    - Fixed the size of the previews to 770x380 pixels. (LP: #1045243)
    - UnityWindow: use smart pointers, use static close_icon (with dynamic
      state) and PanelStyle context (LP: #1033935) (LP: #1045127)
      (LP: #1046124) (LP: #1046126)
    - Remove everything in the #ifndef USE_MODERN_COMPIZ_GL ifdefs and remove
      the ifdefs alltogether. unity now requires compiz...