Mouse wheel zoom of plot also scrolls pane

[sparsonslab]

Description

• Given: a Chaco plot with ZoomTool, inside a scrollable pane.
• When: the user zooms the plot with a mouse-wheel action.
• Then: the pane is simultaneously scrolled.

Possible Origin

@corranwebster had some suggestions:

Possibly a bug in the ZoomTool or Enable. When a tool handles an interaction, it should flag the Enable event object as having been handled (there is a trait on it for that) and then that should also be conveyed to the underlying toolkit.
I suspect that one of these two things is not being done for the mouse wheel event.

Looks like it might be a bug in the enable.qt4.base_window module: according to https://doc.qt.io/qt-5/qwidget.html#wheelEvent it looks like we might need to call event.accept() if the event is handled or event.ignore() if not (which can be detected by the return value from handle_mouse_event.

When the application window height is reduced such that the pane becomes vertically scrollable, using the mouse wheel on a plot can cause two actions to occur simultaneously:

1. The pane scrolls up or down.
2. The intended mouse wheel action for the plot.

leading to a frustrating UX. At present:

• The vertical scrollbar (and thus this bug) only appears when the application window is reduced to less than ~2/3 of the default height. This threshold depends on the layout of each tab in the pane, however, so could change.

Below is a description of what happens from @nicolasap-dm of this occurring

Scrolling the mouse wheel while hovering the pane area now scrolls the area; however, that event is also captured by the ZoomTool when we're hovering one of the tabs. Here, the wires get crossed and we end up scrolling and zooming at the same time.

[rahulporuri]

closing as #552 should have fixed this issue. @sparsonslab if you still see issue with the latest enable (5.0.1), please update/reopen this issue.