I have the same issue, but I think this is not MyPaint's fault, but a buggy implementation of the Wayland protocol on Gnome shell. Whenever I connect my Wacom Intuos tablet to my laptop, a second cursor (which only reacts to the wacom pen) is displayed on the screen.
In this context, I'm experiencing the following behaviour:
- Each cursor works independently of the other: one is controlled by the laptop's touchpad, the other is controlled by the wacom pen. Also, both cursors can be moved across the screen simultaneously.
- The 'wacom-pen-cursor' is not always displayed on the window of some apps (for example, at the time of writing here, the cursor refused to show on the Firefox window for a while).
- The 'wacom-pen-cursor' can't interact with the window of some apps (for example, once the cursor was displayed on the Firefox window, I was unable to change the current tag, select text, or use any of the menu options of the web browser. However, the close, maximize and minimize buttons did work properly).
- Both cursors interact for bad when some actions are performed. For example, as the attached image shows, if both cursors are displayed over a Nautilus (Gnome's file manager) window, and I click inside with the 'wacom-pen-cursor', all the files inside the rectangle determined by both cursors are selected, even if my intention never was to select any file. The same behaviour is experienced with Evince (if you click inside a text area you will end selecting the text covered by the rectangle determined by both cursors). In the particular case of drawing and painting apps, one consequence of this issue is the one you are describing in your post.
I think whe should report a bug to the Gnome develoment team. Maybe, the bug should be reported to Mutter developers (as far as I know, Mutter is the window manager of Gnome Shell).
Basic System Details:
OS: Antergos Linux
Desktop Environment: Gnome Shell, version 3.24.2
Processor: Intel® Celeron(R) CPU N2840 @ 2.16GHz × 2
Tablet: Wacom Intuos, model CTL-480