From 98c8fdcd381e8166724abe3e492bc32012c1bffe Mon Sep 17 00:00:00 2001
From: Botond Ballo <botond@mozilla.com>
Date: Tue, 5 May 2020 17:22:01 +0000
Subject: [PATCH] Bug 1634206 - Change the default value of
 InputAPZContext::sApzResponse to nsEventStatus_eSentinel. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D70084
---
 dom/ipc/BrowserParent.cpp               | 14 ++++++++++++++
 gfx/layers/apz/util/InputAPZContext.cpp |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/dom/ipc/BrowserParent.cpp b/dom/ipc/BrowserParent.cpp
index f9077eeedae3..ce52d5abe278 100644
--- a/dom/ipc/BrowserParent.cpp
+++ b/dom/ipc/BrowserParent.cpp
@@ -3394,6 +3394,20 @@ void BrowserParent::ApzAwareEventRoutingToChild(
     }
     if (aOutApzResponse) {
       *aOutApzResponse = InputAPZContext::GetApzResponse();
+
+      // We can get here without there being an InputAPZContext on the stack
+      // if a non-native event synthesization function (such as
+      // nsIDOMWindowUtils.sendTouchEvent()) was used in the parent process to
+      // synthesize an event that's targeting a content process. Such events do
+      // not go through APZ. Without an InputAPZContext on the stack we pick up
+      // the default value "eSentinel" which cannot be sent over IPC, so replace
+      // it with "eIgnore" instead, which what APZ uses when it ignores an
+      // event. If a caller needs the ability to synthesize a event with a
+      // different APZ response, a native event synthesization function (such as
+      // sendNativeTouchPoint()) can be used.
+      if (*aOutApzResponse == nsEventStatus_eSentinel) {
+        *aOutApzResponse = nsEventStatus_eIgnore;
+      }
     }
   } else {
     if (aOutInputBlockId) {
diff --git a/gfx/layers/apz/util/InputAPZContext.cpp b/gfx/layers/apz/util/InputAPZContext.cpp
index 5085d3fcc1c0..77573221ff38 100644
--- a/gfx/layers/apz/util/InputAPZContext.cpp
+++ b/gfx/layers/apz/util/InputAPZContext.cpp
@@ -11,7 +11,7 @@ namespace layers {
 
 ScrollableLayerGuid InputAPZContext::sGuid;
 uint64_t InputAPZContext::sBlockId = 0;
-nsEventStatus InputAPZContext::sApzResponse = nsEventStatus_eIgnore;
+nsEventStatus InputAPZContext::sApzResponse = nsEventStatus_eSentinel;
 bool InputAPZContext::sPendingLayerization = false;
 bool InputAPZContext::sRoutedToChildProcess = false;