Today I tested this laptop with testing and unstable branch, all defaults, no personalized parameters or configurations.
About lightdm graphical glitches: Couldn't reproduce any hang, frozen frame, or 'black screen' when using testing or unstable branch. (firmware seems to be related, will test stretch with firmware-amd-graphics-20190114 when available), every animation is properly refreshed, from typing to menus. About wallpaper issues: When using testing or unstable branch I couldn't reproduce any graphical glitches in wallpapers (neither cinnamon or lightdm) seems related to firmware as well. About Cinnamon's lockscreen: Still buggy in both testing and unstable, although linux-image-4.19.0-3-amd64 is definitely smoother compared to 4.19.0-2 (stretch,testing), cinnamon's lockscreen doesn't seem to refresh in real time, and it rather refreshes itself every 5 seconds exactly (easy to check with the lockscreen's clock). With stretch it doesn't refreshes at all so it looks like a black screen (like what happens with lightdm/stable), but it's actually running fine if you type the password (avoiding typos while typing blind), it's just that there's no graphical refreshing (to put in in my own terms lol). Since amdgpu.dc it's enabled by default, an old issue with redshift not lowering the cursor's temperature color, it's as well fixed. I do see some weird lines appearing and disappearing in the top right corner of my screen and some other areas, but I guess it's fine since it's unstable branch. This laptop (acer with a12-9720p, radeon rx 540) works awful with linux btw, Acer doesn't care, neither AMD, sadly I might be returning it / selling it, next month. Thanks for the help, sorry for any typos.
