Touchscreen does not work for a tablet connected with an external monitor for 14.04

I have a Samsung Series 7 Slate tablet. Through the HDMI connector on the dock, I connect an external monitor to make a dual-screen setup. The tablet screen (LVDS1) is to the left of the external monitor (HDMI1). The screen layout is attached.

I found the touchscreen does not work with this set up. When touch the tablet screen, the cursor is mapped to the external monitor instead of the tablet screen. This makes the touchscreen unusable. Making the external to the left of LVDS1 results in the same - only the cursor on HDMI1 is controlled by the touchscreen on LVDS1.

My xinput list output is listed below:

fangq@nvwa:~$ xinput list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ Wacom ISDv4 EC Pen stylus id=9 [slave pointer (2)]
⎜ ↳ USB Optical Mouse id=12 [slave pointer (2)]
⎜ ↳ Conexant USB Audio id=14 [slave pointer (2)]
⎜ ↳ Atmel Atmel maXTouch Digitizer id=15 [slave pointer (2)]
⎜ ↳ Wacom ISDv4 EC Pen eraser id=17 [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)]
    ↳ WebCam SC-30AFL11449M id=10 [slave keyboard (3)]
    ↳ WebCam SC-20FHM11347N id=11 [slave keyboard (3)]
    ↳ Dell Dell QuietKey Keyboard id=13 [slave keyboard (3)]
    ↳ AT Translated Set 2 keyboard id=16 [slave keyboard (3)]