XAML Islands: Synaptics touchpads can't scroll XAML content

[zadjii-msft]

Describe the bug

This is being filed to track the following bug in the Windows Terminal: microsoft/terminal#979

I've got a HP Spectre 2017 x360 that I use at home, with a touchpad using the synaptics drivers. On this laptop, I can't use the touchpad to scroll the contents of the Terminal at all. Scrolling simply doesn't on any DPI scaling nor display. However, scrolling with a actual mouse wheel works perfectly fine.

From what I've gathered, nothing in the XAML island ever receives a PointerWheeled event when this touchpad is used to scroll the Terminal. However, we do get a WM_MOUSEWEEL window message on our parent HWND (the HWND that's hosting the XAML island) on this type of device. This WM_MOUSEWEEL does not occur on the Surface Laptop I've also been using to test.

From what I can tell, the Synaptics touchpads don't support "Scroll inactive windows" broadly^[1]. notepad.exe, the Settings app, nothing. Even with "Scroll inactive windows when I hover over them" enabled.

[1]: Additional research discovered that inactive window scrolling works fine on these devices if certain applications are in the foreground. Applications can scroll fine with the touchpad while inactive when Settings is in the foreground, but does not scroll when Chrome is in the foreground. NOTE that the Terminal never scrolls, even when the Terminal is in the foreground.

My working theory, which is derived from notes in #2101 is that the entire XAML island is treated like an inactive window. Unfortunately, I don't really understand the XI input stack, so that might be way off base.

Steps to reproduce the bug

• With a laptop with a Synaptics touchpad, use any version of the terminal <= 0.10.781. (0.11 will likely include a workaround)
• Attempt to scroll the contents of the Terminal window
• Note that it doesn't scroll.

Expected behavior

• The Terminal should scroll.

Version Info

NuGet package version: `Microsoft.UI.Xaml.2.3.191217003-prerelease

I'm seeing this on Microsoft Windows [Version 10.0.19578.1000], RS_ONECORE_DEP, though this has been around for months.

Device form factor	Saw the problem?
Desktop	Yes
Mobile	n/a
Xbox	n/a
Surface Hub	n/a
IoT	n/a

Additional context

I've previously discussed this with @Austin-Lamb and @ocalvo in a mail thread.

[crutkas]

PowerToys also has this bug. microsoft/PowerToys#3400. the microsoft Arc mouse seems to be affected as well

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[zadjii-msft]

This is absolutely still an issue, and I maintain that just documenting this issue isn't a real solution. Internally, this is also discussed in MSFT:37969989, and a fairly long mail thread with some IXP and PowerToys folks.