You misunderstood, this is not about computing individual pixels but only selective rerendering graphical elements which have been changed, and in turn figuring out the total area of change. This propagates through the entire stack to let the GPU scanout hardware know which tiles have changed, and allow partial panel self refresh updates (depending on hardware).

Rendering is still done in bulk for the changed areas, avoiding rendering expensive elements (e.g., transformed video buffers, deeply layered effects, expensive shaders). It's a fundamental part of most UI frameworks.

>you never compute individual pixels because you never need to

Pixel shaders are looking at this laughing at you. PS_OUTPUT is a single pixel whether you want it or not. PS wavefronts are usually very small, so you're still going to be doing a lot of sampling.