Work around zoom issues on Linux.

[asgeirn]

In Linux, the scroll event is sometimes issued with a deltaY of zero. From experience, this seems to occur as often as events including a deltaY value.

The net effect of this, based on how the logic works now, is that zooming in one direction appears to do nothing - as the events cancel each other. In the other direction zooming occurs twice.

Tested and confirmed working on Linux and Windows using JDK 11.