When scrolling with the mouse wheel while pointing within the inbox window, I expect the inbox window to scroll, not the main page.
I'm on macOS+Safari. Sometimes the main window scrolls, sometimes the inbox window scrolls. It is rather unpredictable. Move the mouse a bit and try again usually works. — This is not about the main page scrolling when you get to the end of the window scroll range, it is the window not scrolling when it should.
I had never before tried to scroll the achievements window, but I notice now that it has the same unpredictable behavior. The community window (rightmost button on the top bar) does too.