Touch screens support the two-fingered method of zooming in and out on a web page. That method does not work on the HTML canvas used for PJS programs. To shrink or grow a program the user must move to an area of the web page to perform the operation, and then move back to the canvas.
It would be nice to have that feature pass through the canvas to the underlying web page so that it works for the canvas.
It could be on all the time, or like the Context Menu, it could be made an option controllable from the PJS code. (eg: enableContextMenu() ) which ever is more suitable....
The reason is that the canvas can often be displayed larger than mobile devices can show. The user's intuitive remedy is to use the two-fingered pinch feature to shrink the canvas into view. However, it is not supported and only experienced users would know to move off the canvas to the web page area to perform that operation. It would be nice if the canvas supported that feature from the start.