On Thu, Sep 12, 2013 at 10:42 AM, Ian Hickson <i...@hixie.ch> wrote:

> On Thu, 12 Sep 2013, Robert O'Callahan wrote:
> > On Wed, Sep 11, 2013 at 11:24 AM, Ian Hickson <i...@hixie.ch> wrote:
> > > On Wed, 11 Sep 2013, Robert O'Callahan wrote:
> > > > Pinch-zoom is hard because we don't want to trigger reflows or other
> > > > expensive behavior on pinch-zoom. I'd leave pinch-zoom out of it for
> > > > now.
> > >
> > > Unless I'm missing something fundamental, changing the pixel density
> > > doesn't cause a layout, it's changing the width that causes a layout.
> >
> > Changing pixel density does cause a layout in Gecko. For example we
> > round CSS border widths to a whole number of device pixels. (This
> > ensures that when page zoom is applied, all borders with the same
> > specified width are rendered with the same visual width.) On some
> > platforms (Windows), changing pixel density affects text hinting which
> > affects text layout.
> So how do you do pinch-zoom under the hood on mobile devices?

We don't change layout, we just rerender with transformed coordinates. This
means that pinch-zoom can break apparent border-widths and you can get ugly
hinting on platforms that use pinch-zoom and have fonts with aggressive

We could continue to not change layout, and still change the pixel density
of canvases during or after pinch-zoom. We could even change the pixel
density of canvases inside scaling CSS transforms.

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