Actually, Gecko's compositor knows the screen space invalid region and scissors out the rest when compositing. The invalid region is passed to the window manager. Chromium also does that although they settle for an invalid rect instead of region which is fair. There is an issue on file to do the same with webrender.

In this use case the "painting" a compositor does from layers is copying one big rectangle. That takes next to zero time. That's not at all similar to "painting" where you actually re-render the entire visible portion of the page.

> That's not at all similar to "painting" where you actually re-render the entire visible portion of the page.

Why not?

Painting a page is mostly just copying pixels from textures too.

You don't see a notable difference between [tens of] thousands of glyphs and bits being layered on top of each other and processed through a display list, versus copying over one single rectangle from A to B?

No, I don't. It adds a small amount of vertex shading time. The GPU primitive rasterization machinery is extremely good.

Benchmark it yourself if you don't believe me!