PSA: KDecoration API break in Plasma 6.3
Fractional scaling is hard. Anyone that had the misfortune of working on it knows that… so it won’t surprise a lot of people that it’s not all figured out yet! Today I’ll talk about the fractional scaling problems with KWin’s server side decorations, and why we need to do an API break to fix it.
This is the simplest part. Many decorations have elements that need to be pixel perfect, like outlines that are only a single pixel wide. When they’re not perfectly scaled, or positioned wrongly, that’s sometimes quite visible and annoying:
Logical coordinates are a way to represent the size of something on the screen in a mostly display-independent way and are quite useful for the size and position of things like windows or the cursor. They’re calculated in a really simple way:
With just that equation, there are no problems just yet - you can just multiply the logical coordinate with the display scale, and you get back the original coordinate in pixels. When you round that logical coordinate, and do some calculations with it, things get weird though… let’s look at the concrete example of a window at scale 1.25, and with a 1 pixel wide outline:
