Disable double tap zoom on mobile sites, to remove 300ms click delay

content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java

Index: content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java
diff --git a/content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java b/content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java
index a7da5bc527bc7450f9f1531519e47ec208285838..d973bcab6a1dbc528688296ac2d4b43d1d55ef34 100644
--- a/content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java
+++ b/content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java
@@ -1461,7 +1461,22 @@ public class ContentViewCore
 
     private void onRenderCoordinatesUpdated() {
         if (mContentViewGestureHandler == null) return;
-        mContentViewGestureHandler.updateHasFixedPageScale(mRenderCoordinates.hasFixedPageScale());
+
+        float minScale = mRenderCoordinates.getMinPageScaleFactor();
+        float maxScale = mRenderCoordinates.getMaxPageScaleFactor();
+        float pageScale = mRenderCoordinates.getPageScaleFactor();
+        float contentWidthCss = mRenderCoordinates.getContentWidthCss();
+        float windowWidthDp = pageScale * mRenderCoordinates.getLastFrameViewportWidthCss();
+        // We disable double tap zoom for pages that have a width=device-width
+        // or narrower viewport (indicating that this is a mobile-optimized or
+        // responsive website), as long as the user has not pinch-zoomed in by
+        // more than 10% (in which case they might reasonably expect to be able
+        // to zoom back out using double tap zoom).
+        // It is also disabled for pages that disallow the user from zooming in
+        // or out (even if they don't have a device-width or narrower viewport).
+        boolean disable = (contentWidthCss <= windowWidthDp && pageScale < 1.1f * minScale)

[aelias_OOO_until_Jul13, 2013/10/22 23:43:26]

I don't like the "pageScale < 1.1f * minScale" condition either.  It seems
complex.  Without Blink changes, double-tap won't exclusively go back to
overview mode, it might also zoom in on page elements.  The 10% value also seems
arbitrary.  It seems bound to cause user and developer confusion, I'm not sure
even I'll remember in a month when I can or cannot double-tap.  I'd rather just
have the 100% disable on these sites.

[jdduke (slow), 2013/10/22 23:57:49]

I'd have to agree on this, all or nothing will save a lot of headache here...

[johnme, 2013/10/23 11:46:07]

After thinking about this for a little more, I agree. Done.

+                          || minScale == maxScale;
+        mContentViewGestureHandler.updateShouldDisableDoubleTap(disable);

[jdduke (slow), 2013/10/22 14:15:08]

So the user can double-tap to zoom out, but then double-tap is disabled again? I
wonder if some might find that less than intuitive.  

I think my main issue here is with sites where text auto-sizing is active but we
also have a scalable 'width=device-width' (particulary forums, Reddit, etc...) I
find that there is a LOT of variance in the text sizes for different posts (even
at the same nesting level), making double-tap drag very convenient... I realize
the benefit of tap responsiveness is huge for such sites, but it's a shame to
lose the feature.  

I assume that 'user-scalable-no' results in min=max for page scale, which is how
the current optimization works when they don't explicitly set the min/max?

[johnme, 2013/10/22 15:57:58]

Double-tap toggles between "overview zoom level" and "make the text I tapped on
legible, or zoom to fit if it's e.g. an image".

On pages with a width=device-width (or narrower) viewport, the size of a CSS px
is always >= 1dp, hence the text is already legible (specifically, it's at least
as legible as on desktop, which is our goal), and so double-tap zoom should have
no effect.

Furthermore, users generally don't expect double-tap zoom to have any effect on
width=device-width sites, since the majority of them disable user zooming.

(Of course it's never quite so simple, and in cases where double-tapping
wouldn't otherwise do anything, Chrome for Android currently makes double-tap
zoom in to 1.2x the overview zoom level. But this isn't particularly useful, so
it should be fine to lose this behavior and have double-tap consistently do
nothing on pages with mobile viewports.)

The key point is, that double-tap means "zoom in until this text is legible",
where legible means "scaled such that 1 CSS px >= 1 dp if the text hasn't been
autosized, else zoom-to-fit the enclosing Text Autosizing cluster (i.e. you'll
zoom in less, but that's compensated for by the font size having been made
correspondingly larger)".
Text Autosizing should never be active on width=device-width pages (since, as
mentioned above, the text is already legible).

(Currently, a nasty fudge factor causes us to apply between a *small* amount of
autosizing on width=device-width pages -- http://crbug.com/252828 -- but skobes
and pdr are actively working to remove that.)
Yup.

     }
 
     private void hidePopupDialog() {