Thanks, gordon. That was interesting.
Yes. You should start a blog.
Thanks for this clear explanation and thank you for the time writing it. 
I have compositing (GPU) on and I see exactly the same behaviour of the thumbnail preview.
I thought that wnck-applet was a kind of host for "plugins" like thumbnail-preview which means that any "plugin" can crash the host, but actually the problem is also in the host-code itself becase of a flakey screengrab ?
I would definitely susbscribe if you started a blog.
I like your writing style and and your Xorg/gtk+ knowledge is pretty awesome
About compositors: I like to offload drawing actions to the GPU.
as far as I understood, it saves on CPU cycles and batterytime.
Also , when it fixes screentearing in videos i'm pretty happy that it exists.
If a GPU is not adequate for offloading, or RAM is a bit scarce, I pretty much always switch compositing off.
I have a workable grasp of C (not C++), I'm reasonably ok in bash, but I'm hopeless with X + window managers + graphic libraries + GUI-code so please correct any of my reasoning-errors.