See bug 4066369 for a complete description of the problem.
Here is a description of the problem from that bug report:

This double clicking problem is caused by the non-normal Notify events sent by
X server. For example, when resource SetInput is "select" under OpenWindows, OpenWindows sends additional two non-normal Notify for each mouse click. When
these events get processed in JDK, the static "count" variables in JDK are reset to zeros and will never give clickCount > 1.

The non-normal Notify events should be filtered out, applications and users are not interested in and do not care about these events.