GC Settings painting incorrectly, or Z order issue, on Pi 3B+

I’m running GC 1.26 on a Raspberry Pi 3B+, with a GeChic On-Lap 1502i 1080p cap-touchscreen. There is an issue where tapping the Actions button paints its dialog, but tapping the Settings button does not.

The first time on boot and startup of GC, tapping Settings will paint a mostly-black dialog with some strange notches in white near the top; subsequent launches of Settings either briefly flickers the dialog before disappearing, or won’t show any Settings at all.

All the Settings dialog’s buttons can be tapped, but you cannot see the Settings dialog itself; you are tapping on the unresponsive painted main window UI. You will get a floating dropdown for Maslow/Advanced/Ground Control/Kivy settings if you tap in the top center of the screen, and you can tap on random places in the display to maybe pop up a numeric text entry dialog, and tap on the top right corner of the display to “close” the invisible Settings dialog and get back to the main app.

I suspect a Z-order painting issue but don’t quite know where to start on debugging it.

This reproduces on Raspbian Stretch using these instructions, whether I use GL or SDL: https://github.com/MaslowCNC/GroundControl/wiki/Raspberry-Pi

The exact same repro also occurs on Ubuntu MATE 16.04.2 image, replacing kernel+drivers with Rasbian versions to get it booting on RPi 3B+, then doing updates and raspi-updates to latest.

I wonder if I just go with it, and never use the Settings dialog but instead do all my changes to groundcontrol.ini with a text editor, if I’ll have more issues later? Enough popups seem to work okay such as Z-axis control buttons that I think it will be usable.