Bug 992324 patch 1 - Add a pref to disable interruptible reflow. r=bzbarsky

layout/base/nsLayoutUtils.cpp

@@ -118,6 +118,7 @@ typedef FrameMetrics::ViewID ViewID;
 /* static */ bool nsLayoutUtils::sFontSizeInflationDisabledInMasterProcess;
 /* static */ bool nsLayoutUtils::sInvalidationDebuggingIsEnabled;
 /* static */ bool nsLayoutUtils::sCSSVariablesEnabled;
+/* static */ bool nsLayoutUtils::sInterruptibleReflowEnabled;
 
 static ViewID sScrollIdCounter = FrameMetrics::START_SCROLL_ID;
 
@@ -5678,6 +5679,8 @@ nsLayoutUtils::Initialize()
                                "nglayout.debug.invalidation");
   Preferences::AddBoolVarCache(&sCSSVariablesEnabled,
                                "layout.css.variables.enabled");
+  Preferences::AddBoolVarCache(&sInterruptibleReflowEnabled,
+                               "layout.interruptible-reflow.enabled");
 
   Preferences::RegisterCallback(GridEnabledPrefChangeCallback,
                                 GRID_ENABLED_PREF_NAME);

layout/base/nsLayoutUtils.h

@@ -1870,6 +1870,11 @@ public:
     return sCSSVariablesEnabled;
   }
 
+  static bool InterruptibleReflowEnabled()
+  {
+    return sInterruptibleReflowEnabled;
+  }
+
   /**
    * Unions the overflow areas of all non-popup children of aFrame with
    * aOverflowAreas.
@@ -2121,6 +2126,7 @@ private:
   static bool sFontSizeInflationDisabledInMasterProcess;
   static bool sInvalidationDebuggingIsEnabled;
   static bool sCSSVariablesEnabled;
+  static bool sInterruptibleReflowEnabled;
 };
 
 template<typename PointType, typename RectType, typename CoordType>

layout/base/nsPresContext.cpp

@@ -2578,7 +2578,8 @@ nsPresContext::ReflowStarted(bool aInterruptible)
 #endif
   // We don't support interrupting in paginated contexts, since page
   // sequences only handle initial reflow
-  mInterruptsEnabled = aInterruptible && !IsPaginated();
+  mInterruptsEnabled = aInterruptible && !IsPaginated() &&
+                       nsLayoutUtils::InterruptibleReflowEnabled();
 
   // Don't set mHasPendingInterrupt based on HavePendingInputEvent() here.  If
   // we ever change that, then we need to update the code in

modules/libpref/src/init/all.js

@@ -1904,6 +1904,11 @@ pref("layout.scrollbar.side", 0);
 // pref to stop overlay scrollbars from fading out, for testing purposes
 pref("layout.testing.overlay-scrollbars.always-visible", false);
 
+// Enable/disable interruptible reflow, which allows reflows to stop
+// before completion (and display the partial results) when user events
+// are pending.
+pref("layout.interruptible-reflow.enabled", true);
+
 // pref to control browser frame rate, in Hz. A value <= 0 means choose
 // automatically based on knowledge of the platform (or 60Hz if no platform-
 // specific information is available).