Scroll inertia cancel for Win32

[moffatman]

Generate PointerScrollInertiaCancel event on Win32 when DirectManipulation inertia is prematurely ended. This should correspond to a trackpad touch.

Part of flutter/flutter#106979

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[dkwingsmt]

Can we combine the previous if clause and this one into

Suggested change

	if (previous == DIRECTMANIPULATION_RUNNING) {
	if (previous == DIRECTMANIPULATION_RUNNING) {
	if (during_synthesized_reset_) {
	during_synthesized_reset_ = false;
	} else if (current == DIRECTMANIPULATION_RUNNING) {
	...
	}
	// Reset deltas to ensure only inertia values will be compared later.
	...

(Did I suggest the current way when I first reviewed this file? Sorry if I did.)

[moffatman]

We can't do that since the else if would no longer make any sense (previous and current state will not ever be the same).

[dkwingsmt]

We only compare the previous status but not the current, does it mean if the inertia movement dies out naturally it will also dispatch a cancel event?

[moffatman]

Current should always be DIRECTMANIPULATION_READY whether it dies out naturally or is cancelled. We can only tell if it's cancelled prematurely by seeing if the last event still had meaningful velocity left (hadn't already decelerated to near-zero).

[alexmercerind]

May we show some love to this one?
Thanks a lot for the great work.

I don't know how to explain it, but when I scroll on Windows with my trackpad (post Flutter 3.3.x), it feels sort of "un-natural".

Scrolling abruptly stops or starts scrolling in completely opposite direction when making a swipe on my trackpad while a ListView is "already in scroll inertia motion". I have a precision touch-pad. I'm still on Flutter 3.0.5 because of this. I'm not sure if this will fix it, but once again, thanks!

[dkwingsmt]

LGTM