What is this popup button and can it be implemented on WinAPI?

Question

API

What is this popup button?

Who thread faced with this? I really liked this mechanic, and I want to make it on WinAPI.
Does anyone have infa what it's called and how it can be done on WinAPI?

Answer 1

This window is most likely made in Qt. But, on WinAPI, you can also do this. You make a window without a frame and draw the desired bitmap inside. Borderless Window Example