Haven't confirmed yet, but after a cursory glance it seems like this is completely unused?
https://searchfox.org/mozilla-central/rev/8d43262674d6c6d469b821cca579b1240ebb42a5/widget/windows/nsWinGesture.cpp

Looks like we've been using it. But it's only for Window 7? Then we can remove it. I am totally unsure though.

My vague memory is that these were an early attempt by microsoft to implement touch in their OS. Then implemented a new and improved api in Win8+. It might be an API that was introduced with Windows 7, but still supported (maybe deprecated) with newer versions. I'm not sure about any of this, just recalling from memory. If it's still supported by newer versions of Windows we could still remove it if we don't use it and don't plan to ever use it though.

If we call RegisterTouchWindow, our windows will stop receiving WM_GESTURE:
https://learn.microsoft.com/en-us/windows/win32/wintouch/getting-started-with-multi-touch-messages#registering-to-receive-windows-touch-input

We call RegisterTouchWindow when APZ is enabled and dom.w3c_touch_events.enabled is nonzero. Do we still support configurations that APZ is disabled or dom.w3c_touch_events.enabled is zero?

Thanks for the info. Yeah, in popup windows which don't have any remote contents such as popup menus we don't support APZ at all in the windows.

I am not sure about dom.w3c_touch_events.enabled.

Does bug 1682629 need to block this? Do context menus and non-remote popups benefit from these events? Can't think of a scenario where they would but I could be missing something

I am not sure about dom.w3c_touch_events.enabled.

dom.w3c_touch_events.enabled is disabled on macOS.

Not actively working on this