Bug 1105109 - Put APZ autoscrolling behind a pref. r=kats

For now, the pref is off by default.

MozReview-Commit-ID: BtzmFGIAGey

--HG--
extra : rebase_source : a85b20059ae904cc18241e128974bbe8ee060468

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -135,6 +135,10 @@ typedef GenericFlingAnimation FlingAnimation;
  * a single input block. If set to false, a single input block can only
  * scroll one APZC.
  *
+ * \li\b apz.autoscroll.enabled
+ * If set to true, autoscrolling is driven by APZ rather than the content
+ * process main thread.
+ *
  * \li\b apz.axis_lock.mode
  * The preferred axis locking style. See AxisLockMode for possible values.
  *

gfx/thebes/gfxPrefs.h

@@ -285,6 +285,7 @@ private:
   DECL_GFX_PREF(Live, "apz.allow_checkerboarding",             APZAllowCheckerboarding, bool, true);
   DECL_GFX_PREF(Live, "apz.allow_immediate_handoff",           APZAllowImmediateHandoff, bool, true);
   DECL_GFX_PREF(Live, "apz.allow_zooming",                     APZAllowZooming, bool, false);
+  DECL_GFX_PREF(Live, "apz.autoscroll.enabled",                APZAutoscrollEnabled, bool, false);
   DECL_GFX_PREF(Live, "apz.axis_lock.breakout_angle",          APZAxisBreakoutAngle, float, float(M_PI / 8.0) /* 22.5 degrees */);
   DECL_GFX_PREF(Live, "apz.axis_lock.breakout_threshold",      APZAxisBreakoutThreshold, float, 1.0f / 32.0f);
   DECL_GFX_PREF(Live, "apz.axis_lock.direct_pan_angle",        APZAllowedDirectPanAngle, float, float(M_PI / 3.0) /* 60 degrees */);

modules/libpref/init/all.js

@@ -678,6 +678,7 @@ pref("layers.geometry.d3d11.enabled", true);
 pref("apz.allow_checkerboarding", true);
 pref("apz.allow_immediate_handoff", true);
 pref("apz.allow_zooming", false);
+pref("apz.autoscroll.enabled", false);
 
 // Whether to lock touch scrolling to one axis at a time
 // 0 = FREE (No locking at all)

toolkit/content/widgets/browser.xml

@@ -1091,7 +1091,8 @@
                 return false;
               }
               this.startScroll(data.scrolldir, data.screenX, data.screenY);
-              if (this.isRemoteBrowser && data.scrollId != null) {
+              if (this.isRemoteBrowser && data.scrollId != null &&
+                  this.mPrefs.getBoolPref("apz.autoscroll.enabled", false)) {
                 let { tabParent } = this.frameLoader;
                 if (tabParent) {
                   tabParent.startApzAutoscroll(data.screenX, data.screenY,
@@ -1219,7 +1220,7 @@
               window.removeEventListener("keyup", this, true);
               this.messageManager.sendAsyncMessage("Autoscroll:Stop");
 
-              if (this.isRemoteBrowser) {
+              if (this.isRemoteBrowser && this._autoScrollScrollId != null) {
                 let { tabParent } = this.frameLoader;
                 if (tabParent) {
                   tabParent.stopApzAutoscroll(this._autoScrollScrollId,