Touchpad scrolling inertia (momentum) should stop after pressing modifier key

Usage case:
I use a touchpad on my laptop, and use the two-finger multi-touch scroll. If I flick it, the scrolling possesses its own inertia or momentum, and the window continues scrolling (a feature, presumably). However, if I scroll with the touchpad, then immediately hold Ctrl, the inertia of the scrolling acts with the modifier (making the current page zoom in my DE, i.e. the same effect as ctrl+scroll).

In addition, if I scroll then immediately focus another window, the inertia passes to the second window as well.

Ideally, the scrolling inertia should only apply with the initial modifier key (i.e. nothing), and to the original window. This may be difficult to implement, so perhaps it'd be easier to just stop passing the scrolling action when a modifier key is depressed, or another window is focused.

Reproducible: Always

Steps to Reproduce:
1. scroll quickly with the touchpad
2. stop scrolling,
3. immediately hold Ctrl, or focus another window.

Actual Results:
Ctrl+scroll is passed to the window, or scroll is passed to the second window.

Expected Results:
When the modifier key is pressed, either scrolling should continue (with no modifier passed), or scrolling (i.e. non-modifier) should stop. In the other case, when the second window is focused, scrolling inertia in the first window should continue, or scrolling should just stop.

Originally reported in KDE bugtracker here: https://bugs.kde.org/show_bug.cgi?id=311615
Redirected to X bugtracker here: https://bugs.freedesktop.org/show_bug.cgi?id=38909#c25
Apparently, "clients are expected to handle smooth scrolling themselves, and announce support for that feature to the X server."