Do not prevent default on source of simulated event if simulated event is unhandled

[perliedman]

This allows for example touchstart events to propagate so that they can be used to scroll the map when dragging is disabled. This fixes #5425.

[herrernst]

This unfortunately now has a bad side effect: In Safari on iOS, enabling dragging now results in not only scrolling the map, but also the page. But this "double-scrolling" doesn't happen anymore after you have pinch-zoomed once. See attached video (this branch with default options). I have not found out why this happens, maybe you have an idea.

[perliedman]

@herrernst can you share a code example which exhibits this problem? Also, I don't have any iOS devices, so any help you could provide to examine why this happens would be of great help.

[herrernst]

Okay, that wasn't so easy, but I found the reason: Safari on iOS 10 has a weird behaviour where preventDefault on touchmove events doesn't stop scrolling when the handler is added "later" (like in Draggable, where it is added in the touchstart handler):
http://www.openradar.me/28479522
http://www.openradar.me/28359335

So one thing you could do is to preventDefault() already in the touchstart handler, like how it's done in Map.TouchZoom.
I have done so in my fork, and it seems to work, I hope it has not any unwanted side effects: herrernst@1eadeab

(By the way, if you do have a Mac, you can install Xcode and use the iOS Simulator.)

[perliedman]

Wow, thanks, really nice find! You just have to love these little quirks in certain OS/browser combinations... 😖

I'll have to think for a bit about the potential consequences of your simple and elegant fix. Hopefully we can use it.

[SimonEast]

Did anyone get any further with @herrernst's tweak? We're currently building a map application for the World Health Organisation and we'd love to have iOS scrolling fixed.