User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
Firefox/52.0
Build Identifier: 

Touchpad settings allow two-finger (or edge) scrolling, with an additional
"coasting" setting, which causes scrolling to continue after the fingers have
been lifted from the touchpad.

In some programs (e.g., Firefox), scrolling controls separate behaviors
depending on whether modifier keys like CTRL are held down during the scroll.
In the case of Firefox, CTRL+scroll controls the page zoom level.

The problem is that if you initiate a "coasting" scroll without a modifier key
(i.e., to scroll up or down the page) and then press one before the coasting
has stopped, the application will interpret the combination of the "coasting"
scroll and the modifier key to perform the auxiliary behavior (e.g., zooming in
and out of the page).

Reproducible: Always

Steps to Reproduce:
1. Enable "two-finger scrolling" and "coasting" in KDE's System Settings >
Input Devices > Touchpad > Scrolling settings (I believe this is synaptiks?)
2. Switch to an open Firefox window
3. Rapidly swipe two fingers on the touchpad to initiate a coasting scroll,
then quickly press CTRL
Actual Results:  
The text size of the page increases (or decreases, depending on the direction
of the scroll).

Expected Results:  
The text size of the page should not change.

This is occurring in KDE. Don't know if applies to GNOME. I'm on a Late 2008
Unibody MacBook, though I suspect this problem is hardware-independent.
Disabling coasting removes the problem. To be clear, I'm on a fresh install of
Tumbleweed 13.3 (as specified in /etc/SUSE-brand), but selected "13.2" in the
menu above because "13.3" was not available.

An acceptable workaround is to disable coasting.