This is about an OS-layer API, not a windowing toolkit API. Tying the window to a thread at this layer makes it impossible to have a single-threaded UI application. It is inflexible.
"This is about an OS-layer API, not a windowing toolkit API"
What does that even mean? Of course you can have single-threaded UI applications in Windows, the majority of them are. Have you ever written a Windows application in C or C++?
Sorry. I've read your comment a couple of times and am unable to make any sense of it. Win32 is a (dated, clunky) API for writing GUIs (in part). "OS-layer" is babble in this context.
And yeah, i would stop signing your posts - it comes across as conceited somehow; like each comment is a big deal.
What does that even mean? Of course you can have single-threaded UI applications in Windows, the majority of them are. Have you ever written a Windows application in C or C++?