--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -1279,20 +1279,28 @@ nsEventStatus AsyncPanZoomController::On
     case NOTHING:
     case ANIMATING_ZOOM:
       // May happen if the user double-taps and drags without lifting after the
       // second tap. Ignore the move if this happens.
       return nsEventStatus_eIgnore;
 
     case TOUCHING: {
       ScreenCoord panThreshold = GetTouchStartTolerance();
-      UpdateWithTouchAtDevicePoint(aEvent);
-
-      if (PanDistance() < panThreshold) {
-        return nsEventStatus_eIgnore;
+
+      // We intentionally skip the UpdateWithTouchAtDevicePoint call when the
+      // panThreshold is zero. This ensures more deterministic behaviour during
+      // testing. If we call that, Axis::mPos gets updated to the point of this
+      // touchmove event, but we "consume" the move to overcome the
+      // panThreshold, so it's hard to pan a specific amount reliably from a
+      // mochitest.
+      if (panThreshold > 0.0f) {
+        UpdateWithTouchAtDevicePoint(aEvent);
+        if (PanDistance() < panThreshold) {
+          return nsEventStatus_eIgnore;
+        }
       }
 
       ParentLayerPoint touchPoint = GetFirstTouchPoint(aEvent);
 
       MOZ_ASSERT(GetCurrentTouchBlock());
       if (gfxPrefs::TouchActionEnabled() && GetCurrentTouchBlock()->TouchActionAllowsPanningXY()) {
         // User tries to trigger a touch behavior. If allowed touch behavior is vertical pan
         // + horizontal pan (touch-action value is equal to AUTO) we can return ConsumeNoDefault

--- a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
@@ -275,26 +275,20 @@ function* synthesizeNativeTouchSequences
         synthesizeNativeTouch(aElement, aPositions[i][j].x, aPositions[i][j].y, SpecialPowers.DOMWindowUtils.TOUCH_CONTACT, null, aTouchIds[j]);
         currentPositions[j] = aPositions[i][j];
       }
     }
   }
   return true;
 }
 
-// A handy constant when synthesizing native touch drag events with the pref
-// "apz.touch_start_tolerance" set to 0. In this case, the first touchmove with
-// a nonzero pixel movement is consumed by the APZ to transition from the
-// "touching" state to the "panning" state, so calls to synthesizeNativeTouchDrag
-// should add an extra pixel pixel for this purpose. The TOUCH_SLOP provides
-// a constant that can be used for this purpose. Note that if the touch start
-// tolerance is set to something higher, the touch slop amount used must be
-// correspondingly increased so as to be higher than the tolerance.
-const TOUCH_SLOP = 1;
-function synthesizeNativeTouchDrag(aElement, aX, aY, aDeltaX, aDeltaY, aObserver = null, aTouchId = 0) {
+// Note that when calling this function you'll want to make sure that the pref
+// "apz.touch_start_tolerance" is set to 0, or some of the touchmove will get
+// consumed to overcome the panning threshold.
+function synthesizeNativeTouchDrag(aElement, aX, aY, aDeltaX, aDeltaY, aSlop, aObserver = null, aTouchId = 0) {
   var steps = Math.max(Math.abs(aDeltaX), Math.abs(aDeltaY));
   var positions = new Array();
   positions.push([{ x: aX, y: aY }]);
   for (var i = 1; i < steps; i++) {
     var dx = i * (aDeltaX / steps);
     var dy = i * (aDeltaY / steps);
     var pos = { x: aX + dx, y: aY + dy };
     positions.push([pos]);

--- a/gfx/layers/apz/test/mochitest/helper_basic_pan.html
+++ b/gfx/layers/apz/test/mochitest/helper_basic_pan.html
@@ -12,17 +12,17 @@
 function scrollPage() {
   var transformEnd = function() {
     SpecialPowers.Services.obs.removeObserver(transformEnd, "APZ:TransformEnd", false);
     dump("Transform complete; flushing repaints...\n");
     flushApzRepaints(checkScroll);
   };
   SpecialPowers.Services.obs.addObserver(transformEnd, "APZ:TransformEnd");
 
-  synthesizeNativeTouchDrag(document.body, 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(document.body, 10, 100, 0, -50);
   dump("Finished native drag, waiting for transform-end observer...\n");
 }
 
 function checkScroll() {
   is(window.scrollY, 50, "check that the window scrolled");
   subtestDone();
 }
 

--- a/gfx/layers/apz/test/mochitest/helper_bug1280013.html
+++ b/gfx/layers/apz/test/mochitest/helper_bug1280013.html
@@ -14,34 +14,34 @@ function* test(testDriver) {
   ok(screen.height > 500, "Screen height must be at least 500 pixels for this test to work");
 
   // This listener will trigger the test to continue once APZ is done with
   // processing the scroll.
   SpecialPowers.Services.obs.addObserver(testDriver, "APZ:TransformEnd");
 
   // Scroll down to the iframe. Do it in two drags instead of one in case the
   // device screen is short
-  yield synthesizeNativeTouchDrag(document.body, 10, 400, 0, -(350 + TOUCH_SLOP));
-  yield synthesizeNativeTouchDrag(document.body, 10, 400, 0, -(350 + TOUCH_SLOP));
+  yield synthesizeNativeTouchDrag(document.body, 10, 400, 0, -350);
+  yield synthesizeNativeTouchDrag(document.body, 10, 400, 0, -350);
   // Now the top of the visible area should be at y=700 of the top-level page,
   // so if the screen is >= 500px tall, the entire iframe should be visible, at
   // least vertically.
 
   // However, because of the overflow:hidden on the root elements, all this
   // scrolling is happening in APZ and is not reflected in the main-thread
   // scroll position (it is stored in the callback transform instead). We check
   // this by checking the scroll offset.
   yield flushApzRepaints(testDriver);
   is(window.scrollY, 0, "Main-thread scroll position is still at 0");
 
   // Scroll the iframe by 300px. Note that since the main-thread scroll position
   // is still 0, the subframe's getBoundingClientRect is going to be off by
   // 700 pixels, so we compensate for that here.
   var subframe = document.getElementById('subframe');
-  yield synthesizeNativeTouchDrag(subframe, 10, 200 - 700, 0, -(300 + TOUCH_SLOP));
+  yield synthesizeNativeTouchDrag(subframe, 10, 200 - 700, 0, -300);
 
   // Remove the observer, we don't need it any more.
   SpecialPowers.Services.obs.removeObserver(testDriver, "APZ:TransformEnd", false);
 
   // Flush any pending paints
   yield flushApzRepaints(testDriver);
 
   // get the displayport for the subframe

--- a/gfx/layers/apz/test/mochitest/helper_div_pan.html
+++ b/gfx/layers/apz/test/mochitest/helper_div_pan.html
@@ -12,17 +12,17 @@
 function scrollOuter() {
   var transformEnd = function() {
     SpecialPowers.Services.obs.removeObserver(transformEnd, "APZ:TransformEnd", false);
     dump("Transform complete; flushing repaints...\n");
     flushApzRepaints(checkScroll);
   };
   SpecialPowers.Services.obs.addObserver(transformEnd, "APZ:TransformEnd");
 
-  synthesizeNativeTouchDrag(document.getElementById('outer'), 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(document.getElementById('outer'), 10, 100, 0, -50);
   dump("Finished native drag, waiting for transform-end observer...\n");
 }
 
 function checkScroll() {
   var outerScroll = document.getElementById('outer').scrollTop;
   is(outerScroll, 50, "check that the div scrolled");
   subtestDone();
 }

--- a/gfx/layers/apz/test/mochitest/helper_iframe_pan.html
+++ b/gfx/layers/apz/test/mochitest/helper_iframe_pan.html
@@ -13,17 +13,17 @@ function scrollOuter() {
   var outer = document.getElementById('outer');
   var transformEnd = function() {
     SpecialPowers.Services.obs.removeObserver(transformEnd, "APZ:TransformEnd", false);
     dump("Transform complete; flushing repaints...\n");
     flushApzRepaints(checkScroll, outer.contentWindow);
   };
   SpecialPowers.Services.obs.addObserver(transformEnd, "APZ:TransformEnd");
 
-  synthesizeNativeTouchDrag(outer.contentDocument.body, 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(outer.contentDocument.body, 10, 100, 0, -50);
   dump("Finished native drag, waiting for transform-end observer...\n");
 }
 
 function checkScroll() {
   var outerScroll = document.getElementById('outer').contentWindow.scrollY;
   if (getPlatform() == "windows") {
     // On windows, because we run this test with native event synthesization,
     // Windows can end up eating the first touchmove which can result in the

--- a/gfx/layers/apz/test/mochitest/helper_touch_action.html
+++ b/gfx/layers/apz/test/mochitest/helper_touch_action.html
@@ -15,84 +15,84 @@ function checkScroll(x, y, desc) {
 }
 
 function* test(testDriver) {
   var target = document.getElementById('target');
 
   document.body.addEventListener('touchend', testDriver, { passive: true });
 
   // drag the page up to scroll down by 50px
-  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -50),
       "Synthesized native vertical drag (1), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(0, 50, "After first vertical drag, with pan-y" );
 
   // switch style to pan-x
   document.body.style.touchAction = 'pan-x';
   ok(true, "Waiting for pan-x to propagate...");
   yield waitForAllPaintsFlushed(function() {
     flushApzRepaints(testDriver);
   });
 
   // drag the page up to scroll down by 50px, but it won't happen because pan-x
-  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -50),
      "Synthesized native vertical drag (2), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(0, 50, "After second vertical drag, with pan-x");
 
   // drag the page left to scroll right by 50px
-  yield ok(synthesizeNativeTouchDrag(target, 100, 10, -(50 + TOUCH_SLOP), 0),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 10, -50, 0),
      "Synthesized horizontal drag (1), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(50, 50, "After first horizontal drag, with pan-x");
 
   // drag the page diagonally right/down to scroll up/left by 40px each axis;
   // only the x-axis will actually scroll because pan-x
-  yield ok(synthesizeNativeTouchDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 10, 40, 40),
      "Synthesized diagonal drag (1), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(10, 50, "After first diagonal drag, with pan-x");
 
   // switch style back to pan-y
   document.body.style.touchAction = 'pan-y';
   ok(true, "Waiting for pan-y to propagate...");
   yield waitForAllPaintsFlushed(function() {
     flushApzRepaints(testDriver);
   });
 
   // drag the page diagonally right/down to scroll up/left by 40px each axis;
   // only the y-axis will actually scroll because pan-y
-  yield ok(synthesizeNativeTouchDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 10, 40, 40),
      "Synthesized diagonal drag (2), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(10, 10, "After second diagonal drag, with pan-y");
 
   // switch style to none
   document.body.style.touchAction = 'none';
   ok(true, "Waiting for none to propagate...");
   yield waitForAllPaintsFlushed(function() {
     flushApzRepaints(testDriver);
   });
 
   // drag the page diagonally up/left to scroll down/right by 40px each axis;
   // neither will scroll because of touch-action
-  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -40, -40),
       "Synthesized diagonal drag (3), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(10, 10, "After third diagonal drag, with none");
 
   document.body.style.touchAction = 'manipulation';
   ok(true, "Waiting for manipulation to propagate...");
   yield waitForAllPaintsFlushed(function() {
     flushApzRepaints(testDriver);
   });
 
   // drag the page diagonally up/left to scroll down/right by 40px each axis;
   // both will scroll because of touch-action
-  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -40, -40),
       "Synthesized diagonal drag (4), waiting for touch-end event...");
   yield flushApzRepaints(testDriver);
   checkScroll(50, 50, "After fourth diagonal drag, with manipulation");
 }
 
 waitUntilApzStable()
 .then(runContinuation(test))
 .then(subtestDone);

--- a/gfx/layers/apz/test/mochitest/helper_touch_action_complex.html
+++ b/gfx/layers/apz/test/mochitest/helper_touch_action_complex.html
@@ -52,23 +52,19 @@ function* test(testDriver) {
 
   // Helper function for the tests below.
   // Touch-pan configuration |configuration| towards scroll offset (dx, dy) with
   // the pan touching down at (x, y). Check that the final scroll offset is
   // (ex, ey). |desc| is some description string.
   function* scrollAndCheck(configuration, x, y, dx, dy, ex, ey, desc) {
     // Start with a clean slate
     yield resetConfiguration(configuration, testDriver);
-    // Figure out the panning deltas
-    if (dx != 0) {
-      dx = -(dx + TOUCH_SLOP);
-    }
-    if (dy != 0) {
-      dy = -(dy + TOUCH_SLOP);
-    }
+    // Reverse the touch delta in order to scroll in the desired direction
+    dx = -dx;
+    dy = -dy;
     // Do the pan
     yield ok(synthesizeNativeTouchDrag(scrollframe, x, y, dx, dy),
         "Synthesized drag of (" + dx + ", " + dy + ") on configuration " + configuration);
     yield waitForAllPaints(function() {
       flushApzRepaints(testDriver);
     });
     // Check for expected scroll position
     checkScroll(scrollframe, ex, ey, 'configuration ' + configuration + ' ' + desc);