The Gtk/PrimaryButtonWarpsSlider setting is currently hard-coded to "1" with the comment /* KDE always warps pointer on mouse click */. There are two problems with this. The first is that the setting should be 0, not 1, to better emulate KDE. The 1 setting causes "scroll to the click position" behaviour, which is not what KDE does (see https://developer.gnome.org/gtk3/stable/GtkSettings.html#GtkSettings--gtk-primary-button-warps-slider). If it has to be hard-coded, the value should be 0. The second is that this setting is now settable in the KDE settings tool under System Settings > Application Style > GNOME Application Style (GTK), 'Left mouse button warps scrollbar' and should therefore follow that setting.
kde-gtk-config doesn't use xsettings-kde (does it?)
No :/. ~/kde-gtk-config$ git grep -i xsettings ~/kde-gtk-config$
I didn't know the correct component under which to file this—the problem is actually in xsettings-kde. Feel free to reassign it more appropriately.
*** This bug has been marked as a duplicate of bug 379773 ***