Handling KeyDown events does not prevent TextInput events

Describe the bug In WPF when one handles KeyDown then TextInput event won’t be received. This is useful when handling single character shortcuts and avoiding weird side effects. For example in my app user can rebind their shortcuts, one of such shortcuts focuses an input text box. When such letter shortcut is handled we will mark KeyEventArgs as handled and focus the text box. After text box gets focused it receives text input for the handled letter and immediately types something in it.

Expected behavior I would expect WPF like behavior where handled KeyDown does not produce TextInput.

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Windows
• Version: master

[kekekeks]

It’s more complicated than that.

1. KeyboardDevice doesn’t necessary have the required information to decide if the event should be ignored
2. Depending on the system text input might be decoupled from actual event presses. Sometimes the app won’t see the key press at all

So we should return the “handled” state to the system when possible, but that kinda conflicts with dispatcher priorities.

[grokys]

Ok, so it seems that the way to do this is to not call TranslateMessage if the WM_KEYDOWN is handled, however we have a problem: TranslateMessage is called before the DispatchMessage which dispatches the WM_KEYDOWN to the WindowImpl so we don’t know whether to call TranslateMessage or not until after we’ve called DispatchMessage, at which point it’s too late!

Looks like WPF does this by first calling the OnPreprocessMessage method outside of the WndProc, which handles key down messages and returns the handled status.