fix mouse events in the wpf control

[ZoeyR]

Summary of the Pull Request

PR #4548 inadvertantly broke mouse button input in the WPF control. This happened due to the extra layer of HWND indirection. The fix is to move the mouse button handling down into the native control where the window messages are now being sent.

References

PR Checklist

• Closes #xxx
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place:

Validation Steps Performed

VS was patched with new bits to ensure correct behavior.

[DHowett-MSFT]

I'm surprised the child HWND isn't getting those events, actually. It's not terribly different from a window hosting a button as a child (where the button is just another HWND, ala standard win32/user32.)... I'd be concerned about pursuing a solution based on us not being able to figure out why mouse events aren't going in when keyboard events definitely are.

[ZoeyR]

The child HWND does get the events. The problem was that we were processing these events in the HwndHost which only gets events for the parent HWND