dbltap doesn't work on iPad

[Fil]

Testing d3-zoom (e.g. https://observablehq.com/@d3/zoom-canvas-rescaled or any other) on an iPad, a dbltap doesn't trigger x2 zooming—but a triple tap does. With the pencil it's never triggered, no matter how many "taps".

(On the iPhone dbltap works. )

I'm investigating but it would be nice if someone could confirm or infirm this observation.

[Fil]

• I see the same problem with FF, Chrome, and Safari
• the issue of dbltap being inhibited is present with d3-zoom@1.3.1 but not @1.3.0 (diff)
• adding .style("touch-action", null); makes dbltap operate again
• no versions seem to work with pencil dbltap
• David B. remarks "No issue on BrowserStack's iPad"

[mbostock]

I don’t remember what the point of touch-action: none was…

[Fil]

It looks like the iPad really wants to deduplicate touchstart: on https://observablehq.com/d/a6c2cfc4e1fad9ea as soon as a touchstart listener is registered, a double-tap creates only one event of each kind (touchstart, mousedown, mousemove, mouseup, click and so on).

A solution might be to switch to Pointer Events (https://observablehq.com/@fil/pointer-events), where this doesn't happen. But it looks to me like an iOS (iPad 13.4.1) bug (?).

[Fil]

Re: pencil dbl-tap, the issue is that a touchmove event is received, in which we reset g.taps to 0.

This issue can be fixed by removing:

      if (touchstarting) touchstarting = clearTimeout(touchstarting);
      g.taps = 0;

[Fil]

Another comment on touch-action… #188 (review)

[Fil]

After some digging it appears that touch-action: none was added at about the same time as this conversation happened. In that case it might be related with enabling something (?) on Surface tablets.