Incorrect cursor position scaling (high res. images @ high zoom)
When using drawn mask brushes at high zoom levels on very high resolution images, the brushes do not follow the cursor's real position and are instead scaled incorrectly such that moving the OS-provided pointer to the top of the screen only raises the brush (which duplicates the cursor position) somewhat (eg. 20-30% of the height of the image window). Similarly, moving the OS-provided cursor to the bottom of the screen also only brings the drawn mask cursor a similar distance downward. This makes it impossible to work with the image unless the entire area to be painted is positioned within the center of the image display portion of the window.
Attached is an example showing the bottom-most position of the darktable drawn mask cursor in one instance where the OS-provided cursor has been moved to the very bottom of the screen (and is still visible, just not included in the screenshot due to OSX's builtin screenshot tool's preference for removing it).