A Windows bug with Win32 popup menus and middle-clicking

I was going to start this post by explaining the bug in detail. But, instead, here’s a video of me demonstrating it in Firefox: That’s not the tab I wanted to close Here, I moved Firefox close to the top-left of the screen, but offset slightly to the right. I then pressed Alt+Space to open the system menu. Finally, while that menu was open, I middle-clicked1 on the third tab which had the Firefox website open. Instead of that tab being closed, the BBC News tab to the right of it was. The bug in detail As a bit of background, windows created using the Win32 API (usually using the CreateWindowEx function) receive messages through a function called a window procedure. (That window procedure is specified when the relevant window class is registered.) These messages include mouse events, such as when particular mouse buttons are pressed. Those mouse events are typically supplied with client co-ordinates of where the mouse pointer was at the time of the event. Client co-ordinates mean that…