Region PickOnBounds default setting

[Richard Bair]

Hi folks,

We have an issue which has been in the platform from before 2.0: http://javafx-jira.kenai.com/browse/RT-17024.  A better explanation of the issue can be found on http://javafx-jira.kenai.com/browse/RT-12258. From 12258:

> Region behaves counter-intuitively regarding mouse event delivering. It reacts on mouse events everywhere in its bounds and people are often confused by it. Here are two simple examples: 
> 
> 1) You create let's say HBox just because you want it to layout its children. The HBox catches all mouse events in the whole area given by its bounds. Often it's hard to understand what area it is (with children of different size or with some other layout stretches taking place). 
> 
> 2) You create a small Pane in top-left corner of the scene with a child in bottom-right corner of the scene. Pane's bounds will then cover whole sceen and you won't be able to click on anything else than the pane and its child. Users don't understand why, because both visually and in source code there is nothing in between the pane and the child. 
> 
> Moreover, region may have a shape associated and the behavior here is also strange. If you create a region with a shape inside its bounds, it's just ignored. You can also create a shape somewhere else, then it extends region's bounds and it reacts on mouse click everywhere between the shape and the region. 


This issue has to do with the semantics of picking on a Region. For Region we have had pickOnBounds set to true by default, which yields the above behaviors. We can change it to false by default, but then need to update a bunch of skins (for example the up/down arrows of scroll bar, the thumb of a slider, the down arrow of a combo box button, etc) so that they switch back to having pickOnBounds set to true by default so that the target area for clicks is larger. We could just change the default for Pane and not for Region, although we use StackPane in Skins and would have to update them anyhow. It seems that for a normal layout container the behavior really should be pickOnBounds=false by default, but for UI controls usages and such you generally want it true.

I'm not certain making this change is worth being backwards incompatible (semantically, binary compatibility would remain). But what do you think?

Richard