Can't use two-finger horizontal scrolling

When hovering over document pages, I can't use two-finger horizontal scrolling, it only works if I hover the scrollbar handle on the bottom.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: simple-scan 3.16.1.1-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-13.13-generic 3.19.3
Uname: Linux 3.19.0-13-generic x86_64
ApportVersion: 2.17.1-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Apr 17 12:08:09 2015
MachineType: Dell Inc. XPS 12-9Q33
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.19.0-13-generic.efi.signed root=UUID=f6fef09e-9f1a-40ad-aeaa-7d7a67fb86f2 ro rootflags=subvol=@ splash quiet vt.handoff=7
SourcePackage: simple-scan
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/15/2014
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A06
dmi.board.name: XPS 12-9Q33
dmi.board.vendor: Dell Inc.
dmi.board.version: A06
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.chassis.version: Not Specified
dmi.modalias: dmi:bvnDellInc.:bvrA06:bd04/15/2014:svnDellInc.:pnXPS12-9Q33:pvrA06:rvnDellInc.:rnXPS12-9Q33:rvrA06:cvnDellInc.:ct8:cvrNotSpecified:
dmi.product.name: XPS 12-9Q33
dmi.product.version: A06
dmi.sys.vendor: Dell Inc.