I feel like this is a perfect personification of the "came searching for copper but found gold meme", Sometime in the past I acknowledged that modifying UI elements in a secondary thread caused crashes, I even fixed it, then when I moved my project from computer to computer i must have copied an old file or something, and forgot that that was a no-no. But I digress, not only did this thread solve my problem, it also showed me a way to ditch cURL, and rely solely on QT, as well as circumvent the necessity of the threading in the first place, so thank you all for answering my question and more @Buckwheat / @VRonin /@kshegunov

Yeah, that looks ok.
I wasn't suggesting to remove the flush permanently, just to see that the system background is drawing when you don't flush.

Btw. Keep in mind that the system color can change at any time so if you want to be 100% correct you should either get it every time you render or handle the WM_SYSCOLORCHANGE and WM_THEMECHANGED messages.