[Webkit-unassigned] [Bug 138151] New: Can't reliably detect tap from JavaScript

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=138151

            Bug ID: 138151
           Summary: Can't reliably detect tap from JavaScript
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: Unspecified
                OS: iOS 7.0
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Event Handling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: rbyers at chromium.org

JS libraries may want to detect a tap without using the 'click' event - for example in order to work around the 300ms click delay.

Typically the guidance is something like "look for a touchstart, followed by 0 or more touchmove events that don't move beyond N pixels from the touchstart, then a touchend".  The problem with this is that there's no reliable way to determine the value for 'N'.  If you choose too low than it may be too hard to trigger a tap (compared to the browser's default behavior).  If you choose a value that's too large then you may detect a tap when the browser triggers scrolling.  Since the ideal value can change across different devices / versions of iOS, web code can be brittle to iOS gesture detection changes.

The same problem applies when trying to write code which consumes touch events in one axis but not the other (eg. a horizontal image carousel on a vertically scrolling page).

The problem is a little easier in Chromium due to a touchmove suppressing hack we've got - see https://docs.google.com/a/chromium.org/document/d/12k_LL_Ot9GjF8zGWP9eI_3IMbSizD72susba0frg44Y/edit#heading=h.vluzqx9pk2bm.  But if we can agree on an API to let pages do this properly, I'd ideally like to change chromium's behavior to match Safari's here (suppressing touchmove events is potentially evil).

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