The VNC in the browser does not work well with touch screen laptops and is there specifically with Google Chrome as it takes touch as the default input for noVNC which is what is running on the RaspbberyPi to give the browser interface to the VNC server. Can you try another browser like Firefox. Or you can use VNC viewer: https://www.realvnc.com/download/viewer/ to use it ( Use dex.local:1 as the address in VNC viewer.
This bug is not on our image/software end and is in the implementation of noVNC. If the noVNC instance gets updated in the future and this bug gets removed, we'll update the image too.
Do let us know if this helps.