How to make a “popup” (hint, drop-down, popup) window in Winforms?

Question

How can i make, what i will call, a \"popup\" window\" in WinForms?

---

Since i used my own made-up word \"popup\", let me give examples of this so-called \"p

Answer 1

I was curious as to how combobox dropdowns and menus work, so I did some more research.

There are two basic approaches.

Create the popup as an overlapped window, owned by the main window

This method is required if the popup has embedded controls, or if you want the popup to behave as a modeless dialog.

If the user is going to interact with child controls in the popup window, it must receive activation. (So the various techniques for blocking activation, such as handling WM_MOUSEACTIVATE are red herrings.) And when it receives activation, Windows will deactivate the main window. The fix for this is to send a WM_NCACTIVATE message to the parent to update its visual appearance without changing its activation status. This is the approach used by the .Net ToolStrip, and my other answer illustrates it with code.

Create the popup as a child of the Desktop window

This method is used by combobox dropdowns and (I guess) menus, but you can't embed child controls so it's not widely applicable.

The popup is a child window so it doesn't interfere with activation. It is always on top. Mouse capture is used to detect clicks outside the popup and dismiss it.

However, this isn't straightforward to implement. Activation remains with the main application, so it keeps the focus and receives keystrokes. This seems to preclude embedded controls in the popup because they can't receive focus. The combobox handles this by forwarding keystroke messages to its dropdown list. (Note that menus, combobox dropdowns, etc. are all entirely owner-draw in the sense that they have no embedded windows.)