After testing a bit more, I realized that the thread was constantly re-rendering
the thumbnails, even though I didn't touch the "white-background"
button/argument.
This was not just a completely invisible problem, it actually affected the page
selection (it was very hard to select pages by clicking on them, it was randomly
working, and more often not selecting anything). This is how I realized there
was a problem.
The reason was simply that I was never actually calling g_cond_wait() because of
a first_loop flag I forgot to set.
Note that docs of g_cond_wait() explains that it is possible that "spurious
wakeup" happen. At first I thought I had this issue, which is why this commit
also adds a boolean flag to check after a wakeup, to make sure that I was in the
"condition met" case and not the "spurious wakeup" one.
Even though I realized afterwards the real reason was much more stupid, I still
left this additional check.
Fortunately this issue doesn't seem to affect the 2.10 code. Or to be more
accurate: the continuous render very likely happens there too, yet it doesn't
break page selection interaction with GTK+2 as far as I can see.
1584a9ba50