<AWT Dev> [8] Review request for 8024061: Exception thrown when drag and drop between two components is executed quickly

[anton nashatyrev]

Hello,
     could you please review the following fix:

fix: http://cr.openjdk.java.net/~mcherkas/anton/8024061/webrev.00/ 
<http://cr.openjdk.java.net/%7Emcherkas/anton/8024061/webrev.00/>
bug: https://bugs.openjdk.java.net/browse/JDK-8024061

     Problem:  when doing quick drag'n'drop in X11 the incorrect 
behavior appears in the different forms. One is the exception when 
trying to get 'local Jvm' Transferable from DropTargetListener.

     Reason: on quick drag the following sequence of events may appear: 
(1) mouse pressed on source  -> (2) mouse moved to target in a single 
X11 event -> (3) mouse released. What's happening in that case: on event 
(2) only a drag gesture is recognized and no drag enter is generated 
yet. On event (3) the XDragSourceContextPeer assumes that entered event 
(if it happened) was already generated and the 'local JVM' transferable 
had been already captured by SunDropTargetContextPeer from the static 
field currentJVMLocalSourceTransferable. Thus on event (3) the 
XDragSourceContextPeer posts the additional mouseMove event (which turns 
into DragEnter later) and initiates the cleanup which then resets the 
currentJVMLocalSourceTransferable to null. Thus on DragEnter the 
currentJVMLocalSourceTransferable is already null and the 
SunDropTargetContextPeer appears in the inconsistent state.

     Solution: the event (2) from the example above should not only 
initiate the DnD operation but also be a part of that operation, i.e. 
this event should also appear as a drag motion. For that I propose to 
keep a track of the XMotionEvents in the XDragSourceContextPeer to catch 
the mouse event which initiated the DnD and when a startDrag() is called 
process this event as the first drag motion event.

Thanks!
Anton.