GTK 4, which the author mentions in their post, uses a retained rendering model so that the UI can be rendered on GPU. Widgets snapshot render nodes which are immutable and can even be cached between frames. Those render nodes are diffed to calculate the damage region automatically.

That damage is then used to scissor clip in GL before the render tree is converted to shader commands. The original damage flows through to eglSwapBuffersWithDamage() to limit the screen damage to the diffed areas. Some work is needed to do tiled/threaded processing of the render tree to reduce pathological cases instead of large scissor clips. Given how much a DAW changes per-frame when animating, it's probably close to pathological anyway.

In git, there is support for something akin to swap-chains so that you can use DMA-BUF textures (or similar, such as IOSurface), and specify the damage between each buffer. This is useful for things like virtual machines or other external plumbing where you have a buffer+damage to integrate into GTK's composition pipeline and don't want full-frame renders to result due to gsk_render_node_diff() falling over.