It is somewhat difficult to "double click" with the HTML5 client on iPads, since you are not hitting the same pixels. We could consider avoiding moving the mouse cursor directly after a click, or something like that. Check upstream.
Created attachment 481 [details]
Allow for double click to happen even if the second click is a slight bit off
Fixed in the attached patch file.
Made a pull request to the noVNC project on github:
Fixed in revision 27630.
Updated in revision 27660.
Double-clicking seemed easy enough on an iPad during my tests. Closing.