I just recently noticed that my wallpaper is being scaled unnecessarily on my 4K monitor. When I disable HiDPI in MATE Tweak, the wallpaper looks fine. But when I enable it, the wallpaper scales along with the rest of the interface, which causes the wallpaper to appear blurry. In other words, the wallpaper is essentially being displayed at 1920x1080 instead of 3840x2160, the resolution of my display. I searched the web to see if anyone else had experienced this, and indeed, it has apparently been an issue since 18.04 according to the Github repo. I haven't come across any solution to this, nor have I found one myself. This does not seem to be an issue on other desktop environments than MATE. Ever since I noticed this issue, I cannot unsee it and it is starting to bug me very much. If anybody can offer help or a solution that would be much appreciated.