At now KDE have good build-in support for mouse gestures - they works well: https://userbase.kde.org/System_Settings/Shortcuts_and_Gestures#Mouse_Gestures but they works too invisible to user, so users very often draw bad gestures. Many other mouse gestures apps (EasyStroke, Gesturefy and Foxy Gestures Firefox plugins) draw helper lines on screen when user hold gesture button on mouse and start drawing - those lines shows to user that he start drawing gesture, and display which gesture it drawing now. Please add same mode (as option) in KDE Plasma 5 too, for draw mouse trail lines on screen when mouse key is pressed and system ready to recognize gesture. Good example of implementation mouse gesture trails you can see in so useful, but discontinued EasyStroke application.
Fixed/implemented very recently with https://cgit.kde.org/kwin.git/commit/?id=74994a7fbda0432e7b3773e053e4c8c940f66a53
I doubt the KWin effect is related to mouse gestures.
Oh, my mistake.
As example of implementation for drawing gestures lines on screen you can lookup the very popular Mozilla Firefox extensions: https://addons.mozilla.org/en-GB/firefox/addon/gesturefy/ https://addons.mozilla.org/en-GB/firefox/addon/foxy-gestures/ And maybe this can be implemented using Plasma Desktop Effects?
In Plasma already exists very similar Desktop Effect: Mouse Mark It draws the lines on screen when holding Meta+Shift. Maybe it is not so hard to extend it via adding the capture of KDE Mouse Gestures event?
Seems this is duplicate of pretty old 135097 issue. *** This bug has been marked as a duplicate of bug 135097 ***