Unable to get the extra info in WM_KEYDOWN message in WPF form

[SamBent]

[Port of a servicing fix for .NET 4.7-4.8]

Problem description:
Sending WM_KEYDOWN with "extra info" doesn't work. (WM_KEYUP works).

Actual behavior:
WPF discards the extra information before forwarding the WM_KEYDOWN to the app, or raising the PreviewKeyDown event.

Expected behavior:
The extra information is available.

Minimal repro: