When my Dell System XPS L322X laptop, running Ubuntu 12.04, is connected to the Dell P2314H external monitor (this model does NOT have touch screen), the mouse selects anything it hovers over without actually clicking. Note: monitor connected to laptop's mini display port with a DVI cable and a DVI/mini-display-port adapter.
Other notes: I have an external mouse and keyboard connected to a USB hub which is connected to one of the laptop's two USB 3.0 ports. To avoid hibernating when laptop lid is closed, I set IgnoreLid=true in /etc/UPower/UPower.conf
I looked at Mouse selects everything on its own; Universal Access's Hover Click is OFF. I then checked https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1002788 as well as http://ubuntuforums.org/showthread.php?p=9999160 which suggest it's due to touchscreen (again, this doesn't have touchscreen) and, therefore, to use: xinput set-prop 'NAME OF MY TOUCHSCREEN DEVICE' 'Device Enabled' 0 (after first showing list of input devices using: xinput --list). But ... I don't have touchscreen for this.
Output of xinput --list:
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ USB Optical Mouse id=10 [slave pointer (2)]
⎜ ↳ CyPS/2 Cypress Trackpad id=15 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Power Button id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Power Button id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ USB Keyboard id=11 [slave keyboard (3)]
↳ USB Keyboard id=12 [slave keyboard (3)]
↳ Laptop_Integrated_Webcam_1.3M id=13 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=14 [slave keyboard (3)]
↳ Dell WMI hotkeys id=16 [slave keyboard (3)]
sudo apt-get --purge autoremove xserver-xorg-input-mouse && sudo apt-get install xserver-xorg-input-mouse