The application helloworld.HelloPressDragRelease creates a window of size 600x450.
The screen I am using with the Raspberry Pi has a resolution of 1024x600 and is configured as 1280x800 with the following overlay:

overscan_left=0
overscan_right=256
overscan_top=0
overscan_bottom=200

The application's window is located at 212,50.

I see the following problems:
1. The window reaches to (at least) the right and bottom edges of the screen, even though its maximum X bound should be 812 and its maximum Y should be 500.
2. A tap on the screen is reported as an event with the correct coordinates (verified by physical measurement). However, a tap that should be to the left of the window is interpreted as being inside the window.

It looks like the problems are in the bounds given to dispmanx.

With the mouse cursor the application works as expected. This is probably because the mouse cursor is also a Dispmanx window and has the same bounds problem.
When the mouse cursor is moved to the bottom right of the screen it reports coordinates 812, 445 instead of the expected 1024,600.