Quote:

Apologize for the absence, but I've now managed to obtain conclusive results on this graphical issue involving the 17-inch iMac G5 iSight (model A1144):

TL;DR
The graphical issues with this iMac's ATi Radeon X600 Pro are exclusive to MorphOS, not to Mac OS X or Linux. There is no GPU failure in this iMac, nor did excessive heat and gaming produce graphical artifacts within Mac OS X.

If you wish to follow along with the seven images, then please download them from https://postimg.cc/gallery/Cwpc41h/4c37fa5c. Again, JavaScript is not required to view the images and this link is set to expire in one week.

Long answer
I've installed Mac OS X Leopard from my Leopard retail DVD to the iMac and did what I could do on MorphOS to reproduce the graphical glitches. Restarting the iMac yielded no glitches. In previous posts to this thread I said "glitches may occur once GPU temperature is at least 59 degrees Celsius" and "GPU sitting at 59 C, the catastrophic glitches returned after about 15 minutes, unprompted". The program iStats reports temperatures for a Mac's internals, and it reports that after the reboot needed after installing Mac OS X Leopard, the CPU temp sat at 59 degrees C, the GPU sat at 78 degrees C, and the Northbridge sat at 83 degrees C. Temps in MorphOS reports temperatures for the CPU, GPU, and Northbridge, and the numbers seen here are similar, if not a little higher than they are on MorphOS, and the graphics do not go ballistic after reboots nor with increasing heat as they did in my tests on MorphOS. (see 1-imac-temps-after-install.jpg)

Let's play a game. I chose "Tetris Zone". (see 2-imac-playing-game-hot.jpg). In the photograph, you can see iStats reporting the CPU at 65 degrees C, the GPU at 82 degrees C, and the Northbridge at 85 degrees C. Graphics remain consistent as they should.

Let's play another game. I chose "Duke Nukem 3D". (see 3-imac-gpu-no-failure.jpg). Duke Nukem 3D changes the resolution of the iMac's screen, like BoXXiKoN does in MorphOS. While MorphOS bugged out upon having the iMac's resolution be changed by the game, Mac OS X does not mind having its resolution changed by Duke Nukem 3D.

Let's push the iMac's GPU to its limits with a GPU benchmark like XBench (see 4-imac-testing-gpu-benchmark.jpg). Morphever on MorphOS ramped up the temperatures to the point that graphical artifacts and glitches appeared, but will XBench on Mac OS X do the same? The answer is no (see 5-imac-gpu-benchmark-test.jpg). Apparently I received an overall score of 60.95 (see 6-imac-gpu-result.jpg).

Finally, I tried changing the resolution myself a few times in Mac OS X like I did in MorphOS. Nope, no glitches (see 7-imac-changing-resolutions-no-glitch.jpg). You can see based on the progression of these images that 21 minutes had passed between the first picture taken and the last picture taken, making Mac OS X survive longer than MorphOS's ~15 or so minutes after 1 or 2+ reboots without the graphics obliterating themselves.

I really wanted to dedicate this machine to MorphOS, since that is probably the only point to having an iSight iMac G5 if one does not wish to run Mac OS X on it, considering neither OpenBSD nor NetBSD report compatibility with it. It seems that 17-inch iSight owners are at an even bigger disadvantage, since only the ATi Radeon X600 XT in the 20-inch is confirmed by https://morphos-team.net/ to have compatibility with MorphOS. I believe my tests to have doubly confirmed that at this point now.