Strange touch event on twentytwenty-handle

[StateLibraryofQueensland]

I have been looking through the twentytwenty css and js code and I still can't understand where a particular behavior is coming from.

Anyhow, I have tweaked this plugin so that it does not show the handler in the middle or the arrows - just the separation line between the images. This when I can see a strange behavior happening...

When checking it out on a smartphone, I touch on the center of the image - a little grey translucent box flashes up on the screen (centered both horizontally and vertically). For the life of me I cannot style this (with css) or get rid of it. Of course I can css style the handler with a specific background color but it the 'shadow flash' affect still keeps happening.

Can anyone tell me how to control this behaviour? It almost appears to be some native webkit behaviour or something.