Introduce PopupAsync on UserControl/Form

[KlausLoeffelmann]

Rationale

Since Windows 8, Windows introduced a popup-like Dialog for imfo messages or short input actions, which gets dismissed just by clicking next to the Dialog. With that, it has a classic popup (menu) behavior.

Like this:

Currently, there are no means to utilize these kinds of Dialogs in WinForms. Together with #5917 and #4631, this API is not only an important extension for programming with the typical .NET async/await pattern - it also solves 2 requirements in modern, desktop UI design:

1. It makes Windows 8, 10, 11-typical popup-like Dialogs possible.
2. It is an easy extension for Popup-Controls which custom content.

Proposed APIs:

var popupResult = await UserControl1.PopupAsync([IWin43Window owner]);

To show a UserControl in a popup-like Dialog style.

var popupResult = await UserControl1.PopupAsync(popupInitiateControl control);

To show a UserControl as a real popup to a parent control (like a TextBox with an attached down button to open the popup context). In this case, we should provide functionality, to attach the popup visually in a meaningful way to the parent control, to give the visual impression of both UI parts as a unit.

[KlausLoeffelmann]

Locked this for now, since the concept is still work in progress.

[KlausLoeffelmann]

Unlocked. Let's go from here.

[tbolon]

Is this popup intended to be a "layer" (with a mask) on top of the owner form / control ? Or an independent "window" ?

The popup used on windows is a real window, which seems to capture the focus and is dismissed automatically when clicking anywhere outside. It can expand outside the app boundaries.

[KlausLoeffelmann]

In my mind: The latter. 😄
But I'd need to investigate more closely. The idea also is to have a dropdown mechanisam, so we could host UserControl content in a dropdown. Like the Data Binding Picker does.

Sorry for the late response.

[janseris]

Why does this use async/await function call? What would be the difference if it didn't?