Hi, I have some experience on the Linux+Wayland side here and can hopefully provide a little background.
On Linux+Wayland, the "OS" (compositor, usually using libinput) is not responsible for scroll inertia/momentum. It merely creates and delivers `wl_pointer.axis`{`_stop`,`_source`,} and `wl_pointer.frame` events. The axis events are in units of client pixels. Some clients (e.g. GTK3 ones and firefox) implement scroll inertia, while others (e.g. the `foot` terminal) do not. All clients receive the same events from the compositor for the same finger motions.
On Windows, I'm less sure how things work, but after some reading this is my current understanding: it [seems](https://pavelfatin.com/scrolling-with-pleasure/#windows) that trackpad drivers themselves may implement momentum (which is the only place it could be done if their only interface with applications is `WM_MOUSEWHEEL` messages). There aren't separate smooth scroll events, but by using deltas of less than `WHEEL_DELTA` (120), scrolls of less than one mousewheel notch may be transmitted. Apparently `WHEEL_DELTA` should move the scrolled object by `WheelScrollLines` (by default 3).
This means we have "pixels" on the Wayland input event side and "scrolls of 3 lines" on the Windows side. I don't know how to convert between these other than by saying that a line is probably between 10 and 50 pixels depending on font, font size, and DPI. Since the factor we would use here comes prior to any concrete font or display, we probably do have to pick a magic constant (possibly 10; see "further reading" below) to relate pixels and lines. On the other hand, maybe we can reverse-engineer what Libinput on the Wayland side does to get something *not* in pixels, and then see what trackpad drivers on Windows do to convert *that* into something in terms of `WHEEL_DELTA`.
Because WINE is a Wayland client (like Firefox) and (from Windows programs' standpoint) the Windows trackpad driver, does it seem like we should implement trackpad scroll inertia in WINE. That said, it might be best to first land rough support for smooth scroll events before investigating Windows trackpad drivers and Wayland clients (see `gtkeventcontrollerscroll.c` in GTK) to see what algorithms they use.
For further reading, see https://lists.freedesktop.org/archives/wayland-devel/2019-April/040377.html