Show activity indicator while the main thread is blocked (continue)

Question

Continue with previous question I want to be able to show some activity indicator even if the main thread is blocked. (based on this article).

Problems

Answer 1

Again, the only threadsafe way to draw on a window is to draw from the same thread that created a window; anything else is unsafe.

As a possible explanation why your code worked well with old Windows versions and does not work with modern versions read this The Old New Thing article.