I do not know, I only used your example, then checked the code.
Stupid question:
do you follow the rules while creating your custom cursor ?
(especially the size)
I checked there:
https://documentation.xojo.com/api/user_interface/desktop/mousecursor.html
and I do not saw any size, so I suspect some are tempted to use the size they want.
A cursor 50 x 50 ?
Also, the ListBox may be able to change the MouseCursor when it is located in the Heading area (for changing a column width)…
Not the above:
App.MouseCursor = System.Cursors.HandOpen
behave the same in your project.
Looks like a bug. Someone can confirm that ?