Comment 4 for bug 1257153

I have a similar issue with my click pad (Dell XPS 15). The issue is well described in the upstream project there: https://bugs.freedesktop.org/show_bug.cgi?id=66532

The patch suggested by one maintainer (Kit Westneat) fixes the issue as far as I am concerned, and I can finally keep my fingers on the buttons without blocking the mouse movement done with the other finger.

I attach the (modified) patch to add to the package to solve the issue.

To test:
apt-get source xserver-xorg-input-synaptics
cp path/to/fix_multitouch_outside_area.patch xserver-xorg-input-synaptics-1.8.2/debian/patches/
echo fix_multitouch_outside_area.patch >> xserver-xorg-input-synaptics-1.8.2/debian/patches/series
cd xserver-xorg-input-synaptics-1.8.2
dpkg-buildpackage
cd ..
sudo dpkg -i xserver-xorg-input-synaptics_1.8.2-1ubuntu3_amd64.deb

reboot and tell synaptics to ignore the bottom clickpad area:
xinput set-prop 12 "Synaptics Area" 0, 0, 0, 800

Result:
I can now keep my left thumb on the bottom left button while moving the cursor with the right hand.