The position of the visible chunks also changes. I have "Triplebuffer" and "TearFree" enabled in my Xorg.conf for my intel iGPU.
The transparency is due to my camera not being fast enough to catch individual tears in the window. I don't have a compositor enabled, so transparency wouldn't make sense.
Enabling a compositor causes the tearing to stop, but the flashing remains. The compositor reports these errors while Celeste is running:
error 3: BadWindow (invalid Window parameter) request 2 minor 0 serial 87112
error 3: BadWindow (invalid Window parameter) request 20 minor 0 serial 87113
error 3: BadWindow (invalid Window parameter) request 2 minor 0 serial 87114
error 3: BadWindow (invalid Window parameter) request 20 minor 0 serial 87115
error 3: BadWindow (invalid Window parameter) request 2 minor 0 serial 206317
error 3: BadWindow (invalid Window parameter) request 20 minor 0 serial 206318
Picom gives more descriptive errors:
[ 04/12/21 14:38:19.265 x_create_picture_with_pictfmt_and_pixmap ERROR ] failed to create picture (X error 9 DRAWABLE request 138 minor 4 serial 2346)
[ 04/12/21 14:38:19.265 paint_one ERROR ] Window 0x00600014 is missing painting data.
[ 04/12/21 14:38:21.384 x_create_picture_with_pictfmt_and_pixmap ERROR ] failed to create picture (X error 9 DRAWABLE request 138 minor 4 serial 12077)
[ 04/12/21 14:38:21.385 paint_one ERROR ] Window 0x0060005d is missing painting data.
Other OpenGL apps, such as GLXGears, work fine.