Bug 1643508 - Disable touch-based pinch zooming on pdf.js.

Currently using a touchscreen with pdf.js doesn't work so well. In Firefox,
with apz.allow_zooming = false (default on current release/beta), it does a
reflow zoom which makes the UI elements bigger. And with apz.allow_zooming = true
(default on current Firefox nightly), or in Chrome, it does a smooth pinch-zoom
but that also scales up the entire UI. Neither of these is a particularly good
experience, so this patch just disables any multi-touch gestures. Touch-based
panning (which involves a single touch point) is left unaffected.

web/app.js

@@ -1760,6 +1760,9 @@ const PDFViewerApplication = {
 
     window.addEventListener("visibilitychange", webViewerVisibilityChange);
     window.addEventListener("wheel", webViewerWheel, { passive: false });
+    window.addEventListener("touchstart", webViewerTouchStart, {
+      passive: false,
+    });
     window.addEventListener("click", webViewerClick);
     window.addEventListener("keydown", webViewerKeyDown);
     window.addEventListener("keyup", webViewerKeyUp);
@@ -1822,6 +1825,9 @@ const PDFViewerApplication = {
 
     window.removeEventListener("visibilitychange", webViewerVisibilityChange);
     window.removeEventListener("wheel", webViewerWheel, { passive: false });
+    window.removeEventListener("touchstart", webViewerTouchStart, {
+      passive: false,
+    });
     window.removeEventListener("click", webViewerClick);
     window.removeEventListener("keydown", webViewerKeyDown);
     window.removeEventListener("keyup", webViewerKeyUp);
@@ -2519,6 +2525,20 @@ function webViewerWheel(evt) {
   }
 }
 
+function webViewerTouchStart(evt) {
+  if (evt.touches.length > 1) {
+    // Disable touch-based zooming, because the entire UI bits gets zoomed and
+    // that doesn't look great. If we do want to have a good touch-based
+    // zooming experience, we need to implement smooth zoom capability (probably
+    // using a CSS transform for faster visual response, followed by async
+    // re-rendering at the final zoom level) and do gesture detection on the
+    // touchmove events to drive it. Or if we want to settle for a less good
+    // experience we can make the touchmove events drive the existing step-zoom
+    // behaviour that the ctrl+mousewheel path takes.
+    evt.preventDefault();
+  }
+}
+
 function webViewerClick(evt) {
   // Avoid triggering the fallback bar when the user clicks on the
   // toolbar or sidebar.