diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index de8405d15f9f..24b981dcbf4c 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -3921,6 +3921,14 @@ async function synthesizePlainDragAndDrop(aParams) {
       dragEvent
     );
     srcSession.setDragEndPointForTests(event.screenX, event.screenY);
+    if (logFunc) {
+      logFunc(
+        `dragend event client (X,Y) = (${event.clientX}, ${event.clientY})`
+      );
+      logFunc(
+        `dragend event screen (X,Y) = (${event.screenX}, ${event.screenY})`
+      );
+    }
   } finally {
     await new Promise(r => setTimeout(r, 0));
 
diff --git a/widget/nsBaseDragService.cpp b/widget/nsBaseDragService.cpp
index b9566a94dc2a..5dc79af985dc 100644
--- a/widget/nsBaseDragService.cpp
+++ b/widget/nsBaseDragService.cpp
@@ -285,8 +285,8 @@ uint32_t nsBaseDragSession::GetEffectAllowedForTests() {
   return mEffectAllowedForTests;
 }
 
-NS_IMETHODIMP nsBaseDragSession::SetDragEndPointForTests(int32_t aScreenX,
-                                                         int32_t aScreenY) {
+NS_IMETHODIMP nsBaseDragSession::SetDragEndPointForTests(float aScreenX,
+                                                         float aScreenY) {
   MOZ_ASSERT(mDoingDrag);
   MOZ_ASSERT(mSourceDocument);
   MOZ_ASSERT(mSessionIsSynthesizedForTests);
@@ -298,7 +298,7 @@ NS_IMETHODIMP nsBaseDragSession::SetDragEndPointForTests(int32_t aScreenX,
   if (NS_WARN_IF(!pc)) {
     return NS_ERROR_FAILURE;
   }
-  auto p = LayoutDeviceIntPoint::Round(CSSIntPoint(aScreenX, aScreenY) *
+  auto p = LayoutDeviceIntPoint::Round(CSSPoint(aScreenX, aScreenY) *
                                        pc->CSSToDevPixelScale());
   // p is screen-relative, and we want them to be top-level-widget-relative.
   if (nsCOMPtr<nsIWidget> widget = pc->GetRootWidget()) {
@@ -1230,6 +1230,14 @@ nsBaseDragService::SetNeverAllowSessionIsSynthesizedForTests(bool aNeverAllow) {
   return NS_OK;
 }
 
+void nsBaseDragSession::SetDragEndPoint(
+    mozilla::LayoutDeviceIntPoint aEndDragPoint) {
+  mEndDragPoint = aEndDragPoint;
+  MOZ_DRAGSERVICE_LOG("SetDragEndPoint (x,y)=(%d,%d)",
+                      static_cast<int>(mEndDragPoint.x),
+                      static_cast<int>(mEndDragPoint.y));
+}
+
 NS_IMETHODIMP
 nsBaseDragSession::SetDragEndPoint(int32_t aScreenX, int32_t aScreenY) {
   SetDragEndPoint(LayoutDeviceIntPoint(aScreenX, aScreenY));
diff --git a/widget/nsBaseDragService.h b/widget/nsBaseDragService.h
index a735c64430f8..db4b9917bbfe 100644
--- a/widget/nsBaseDragService.h
+++ b/widget/nsBaseDragService.h
@@ -64,12 +64,10 @@ class nsBaseDragSession : public nsIDragSession {
   int32_t TakeChildProcessDragAction();
 
   void SetDragEndPoint(nsIntPoint aEndDragPoint) {
-    mEndDragPoint =
-        mozilla::LayoutDeviceIntPoint::FromUnknownPoint(aEndDragPoint);
-  }
-  void SetDragEndPoint(mozilla::LayoutDeviceIntPoint aEndDragPoint) {
-    mEndDragPoint = aEndDragPoint;
+    SetDragEndPoint(
+        mozilla::LayoutDeviceIntPoint::FromUnknownPoint(aEndDragPoint));
   }
+  void SetDragEndPoint(mozilla::LayoutDeviceIntPoint aEndDragPoint);
 
   uint16_t GetInputSource() { return mInputSource; }
 
diff --git a/widget/nsIDragSession.idl b/widget/nsIDragSession.idl
index d02c6e180b9b..e8a82ef4df37 100644
--- a/widget/nsIDragSession.idl
+++ b/widget/nsIDragSession.idl
@@ -170,9 +170,9 @@ interface nsIDragSession : nsISupports
 
   /**
    * Sets drag end point of synthesized session when the test does not dispatch
-   * "drop" event.
+   * "drop" event.  Position is relative to screen in CSS coordinates.
    */
-  void setDragEndPointForTests(in long aScreenX, in long aScreenY);
+  void setDragEndPointForTests(in float aScreenX, in float aScreenY);
 
   /**
    * Returns true if the session is for dragging text in a text in text control