Close panel if clicked outside

I'm making a custom UserControl (which is compiled to a .DLL, and can be dragged'n dropped from the Toolbox onto forms, just like any other Winforms control). The control is a Combobox. I want to make the Combobox's dropdown (which is a panel, created at runtime) close when the user clicks outside of it, just like a normal ComboBox. And what is the problem? Detecting clicks outside of the panel.

-> Using LostFocus event: Not enough. If the user clicks on a blank space on the form, LostFocus is not fired. I know I can place a LostFocus event on my form and set the ActiveControl to Nothing, but that would require that users (devs) of my custom ComboBox place the same code on every form they use the control on.

-> Using PrefilterMessage: Well, tbh, I don't think I understood how PreFilterMessages works, and I wasn't able to make much progress with it.

-> Using Capture property: Has given the best results so far. There are still some problems though. When Capture is set to True, the mouse is indeed captured by the control which has that property set to True. Therefore, no mouse activity is sensed by any other control on the app, but only the control which has the mouse captured. It'll only release the mouse when the user performs a click. This leads to some problems:

• If the user clicks on an item (a Button) on the dropdown, and the dropdown (a Panel) has its Capture property set to true, the click will be "ignored", as it'll be handled by the dropdown, and not by the Button on which the user actually wanted to click.
• If the user clicks on the dropdown's scroolbar, the click will also be "ignored", just like explained above.
• When user moves the mouse above the Buttons inside the dropdown (the ComboBox's items), they are not highlighted, because no MouseEnter is fired for them, as the mouse is Captured by the dropdown.

There is a way to solve the first issue: You can find on which button is the mouse pointing using Control.MousePosition, and then force a click on it. I haven't been able to find a way to solve the 2nd and 3rd issue though. One possible solution I thought of was setting the dropdown's Capture property to False when the mouse enters on it. So, this is how it would work:

• User clicks on the Combobox. Dropdown opens, captures the mouse;
• User then moves the mouse inside Combobox's dropdown. Capture is set to false, thus making it possible for the user to click on any button inside the dropdown or on the dropdown's scroolbar, and so forth. Buttons inside the dropdown are also properly highlighted as user moves the mouse above them;
• User moves the mouse outside Combobox's dropdown, Capture is again set to true, and now any click the user perform outside of it will be "ignored", and the dropdown will be closed. Just like a normal Combobox.

But when I tried to do this, another issue arrived: when a control has its Capture property set to True, it'll constantly fire MouseEnter events. MouseEnter event doesn't use the real mouse pointer location. Even if the mouse pointer is actually outside a Control, that Control will think the mouse is actually inside of it if its Capture property is set to True.


Thanks in advance, Andre.