As far as I know, it isn't really rendered by HW, but composited in HW. GPUs usually have multiple "Hardware planes" that are composited together in hardware b

As far as I know, it isn't really rendered by HW, but composited in HW. GPUs usually have multiple "Hardware planes" that are composited together in hardware before sending the signal. The OS is free to put whatever it wants there. Most HW include a designated "cursor plane", but it could be used to display whatever. IIRC a minimum of 3 HW planes is specified somewhere, it could be the Wayland protocol, or the Linux Direct Rendering Manager API. Some devices can have less though. That would be the case for that display, which could use an actual framebuffer driver (although planes could be emulated). You can thus have a layer for the background, one for the windows, and one for the cursor, and avoid re-painting too often, even though it should be pretty cheap for GPU-accelerated surfaces (like with compositors that rely on OpenGL).