--- a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
@@ -179,17 +179,26 @@ function synthesizeNativeMouseMoveAndWai
 // relative to the top-left of |aElement|'s bounding rect.
 function synthesizeNativeTouch(aElement, aX, aY, aType, aObserver = null, aTouchId = 0) {
   var pt = coordinatesRelativeToWindow(aX, aY, aElement);
   var utils = SpecialPowers.getDOMWindowUtils(aElement.ownerDocument.defaultView);
   utils.sendNativeTouchPoint(aTouchId, aType, pt.x, pt.y, 1, 90, aObserver);
   return true;
 }
 
-function synthesizeNativeDrag(aElement, aX, aY, aDeltaX, aDeltaY, aObserver = null, aTouchId = 0) {
+// 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) {
   synthesizeNativeTouch(aElement, aX, aY, SpecialPowers.DOMWindowUtils.TOUCH_CONTACT, null, aTouchId);
   var steps = Math.max(Math.abs(aDeltaX), Math.abs(aDeltaY));
   for (var i = 1; i < steps; i++) {
     var dx = i * (aDeltaX / steps);
     var dy = i * (aDeltaY / steps);
     synthesizeNativeTouch(aElement, aX + dx, aY + dy, SpecialPowers.DOMWindowUtils.TOUCH_CONTACT, null, aTouchId);
   }
   synthesizeNativeTouch(aElement, aX + aDeltaX, aY + aDeltaY, SpecialPowers.DOMWindowUtils.TOUCH_CONTACT, null, aTouchId);

--- a/gfx/layers/apz/test/mochitest/helper_basic_pan.html
+++ b/gfx/layers/apz/test/mochitest/helper_basic_pan.html
@@ -12,18 +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", false);
 
-  const TOUCH_SLOP = 1;
-  synthesizeNativeDrag(document.body, 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(document.body, 10, 100, 0, -(50 + TOUCH_SLOP));
   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_div_pan.html
+++ b/gfx/layers/apz/test/mochitest/helper_div_pan.html
@@ -12,18 +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", false);
 
-  const TOUCH_SLOP = 1;
-  synthesizeNativeDrag(document.getElementById('outer'), 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(document.getElementById('outer'), 10, 100, 0, -(50 + TOUCH_SLOP));
   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
@@ -13,18 +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", false);
 
-  const TOUCH_SLOP = 1;
-  synthesizeNativeDrag(outer.contentDocument.body, 10, 100, 0, -(50 + TOUCH_SLOP));
+  synthesizeNativeTouchDrag(outer.contentDocument.body, 10, 100, 0, -(50 + TOUCH_SLOP));
   dump("Finished native drag, waiting for transform-end observer...\n");
 }
 
 function checkScroll() {
   var outerScroll = document.getElementById('outer').contentWindow.scrollY;
   is(outerScroll, 50, "check that the iframe scrolled");
   subtestDone();
 }

--- a/gfx/layers/apz/test/mochitest/helper_touch_action.html
+++ b/gfx/layers/apz/test/mochitest/helper_touch_action.html
@@ -10,90 +10,89 @@
   <script type="application/javascript">
 
 function checkScroll(x, y, desc) {
   is(window.scrollX, x, desc + " - x axis");
   is(window.scrollY, y, desc + " - y axis");
 }
 
 function* test(testDriver) {
-  const TOUCH_SLOP = 1;
   var target = document.getElementById('target');
 
   document.body.addEventListener('touchend', testDriver, { passive: true });
 
   // drag the page up to scroll down by 50px
-  yield ok(synthesizeNativeDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
       "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 waitForAllPaints(function() {
     flushApzRepaints(testDriver);
   });
 
   // drag the page up to scroll down by 50px, but it won't happen because pan-x
-  yield ok(synthesizeNativeDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 100, 0, -(50 + TOUCH_SLOP)),
      "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(synthesizeNativeDrag(target, 100, 10, -(50 + TOUCH_SLOP), 0),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 10, -(50 + TOUCH_SLOP), 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(synthesizeNativeDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
      "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 waitForAllPaints(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(synthesizeNativeDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 10, 10, (40 + TOUCH_SLOP), (40 + TOUCH_SLOP)),
      "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 waitForAllPaints(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(synthesizeNativeDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
       "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 waitForAllPaints(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(synthesizeNativeDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
+  yield ok(synthesizeNativeTouchDrag(target, 100, 100, -(40 + TOUCH_SLOP), -(40 + TOUCH_SLOP)),
       "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);