On smartphones and other devices with small screens, it's essential to be able to zoom into and scroll around images in order to see details. For instance, Wikipedia has many maps whose legend and embedded labels are illegible on a typical smartphone if zoomed all the way out. "Mobile" browsers universally support gestures for zooming and scrolling (usually one-finger touch and swipe to scroll, two-finger "pinch" / "stretch" to zoom).
If you try to use these gestures on a mobile browser displaying an image in the MultimediaViewer, they do not work correctly. I've seen three different, equally incorrect behaviors:
- On an iPhone, the zoom gesture did nothing at all (and it was therefore impossible to test the scroll gestures).
- On Android (4.2) stock browser, zooming enlarges the image but also enlarges the white bar at the bottom of the screen (in general, "chrome" like this should be exempted from zooming) and the image is also displaced downward (the image should not *move*, it should just *expand* under the user's fingers). Worse, after zooming in, scrolling gestures had no effect (so it was impossible to see anything but the top left hand corner of the image).
- Firefox for Android behaves the same as the Android stock browser, except that the white bar at the bottom of the screen *migrates away from the bottom of the screen* as well as enlarging.
On a Commons image-description page, by contrast, this works fine; the gestures apply to the entire page, and the image is embedded in the page normally, so the image is zoomable and scrollable.
I personally consider this a bug of sufficient severity to warrant disabling the extension on all Wikipedias until it is fixed. There are a *lot* of mobile users, and their numbers are growing.