A bonus tangent:

> there's never a point under normal rendering where your entire screen-contents are flattened into a single (externally-addressable) VRAM texture.

...although there used to be, back at the dawn of GPUs. The first "3D acceleration cards" didn't do any 2D drawing — and so PCs would actually have distinct 2D and 3D video cards!

The oldest 2D video cards for PCs, were just persistent framebuffers — their "VRAM" being just for what was on the screen. They relied on the OS to find anything that had been uncovered, and tell the app to repaint it.

But the last and greatest 2D video cards, from before 3D took over, were masters of copying memory around. Each frame, they'd be doing all of these huge accelerated rectangular blits of windows and their controls — some VRAM-to-VRAM, some from main memory using DMA — to bake down a VRAM buffer. They were hardware-accelerated 2D compositors, and they were great at it.

When early 3D cards came along, all that changed at first, was that your OS would tell your 2D card that there was a certain region it was to avoid drawing anything on. The 2D card would leave a black rectangle there.

The 2D card would render and rasterize its VRAM to VGA — and the 3D accelerator card would then be passed this same VGA signal, along a cable, as if it was a display. It would mostly just repeat everything it saw. But when the signal hit the timings for where those black rectangles should appear, then it would instead emit its rendering of the 3D content. (In other words: early 3D cards were https://en.wikipedia.org/wiki/Genlock cards!)

A "hardware-accelerated draw context", in this very early conception of the term, was precisely the geometry, defined by your OS, that your 2D card knew to not bother to draw; which was also the geometry that your 3D card knew was its to draw on.

2D and 3D video cards later merged, and at first you just got one card that did both of these things, as separate phases, with the bridging now internal. But the concept of an HW-accel context lived on.

But later on, actual "GPUs" emerged — in the form of 3D hardware that was now sorta-okay at doing VRAM-to-VRAM copies, and also provided legacy VGA BIOS emulation. So people ditched their 2D cards.

But this change actually kind of sucked, because it meant that all the per-frame 2D blitting (which, again, GPUs were just barely passable at) was now contending for GPU time with 3D stuff. And modern OSes had grown highly dependent on being able to do lots of 2D blitting each frame.

This was why "full-screen mode" was, for the longest time, the way to get the best 3D performance out of a GPU, even at the same display resolution. If you tell the GPU that you want to cover the entire screen with a 3D hardware-accelerated draw context... then it will realize that it'd be pointless to keep drawing the "2D world", and so it will stop spending its time each frame doing 2D VRAM copies to render the 2D-composited layer. It takes the whole OS window-drawing VRAM-copy command-queue out of the loop.

(And also — once there was OS support for this — switching to full-screen mode would also cause the GPU to flush out all the VRAM textures for rendered windows, fonts, icons, etc — giving a 3D app or game far more VRAM to work with. Coming out of full-screen mode would likewise ping the OS to reload all the system brushes and textures into VRAM; and then ping all the running apps to redraw themselves.)