From 74c6c585ab519e3314bdf6b0fa23d8aa757b78a5 Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@gmail.com>
Date: Fri, 24 Aug 2018 13:21:17 +0200
Subject: Introduce ui.menu.allow_content_scroll

When true, overrides the OS convention to prevent scrolling of content when contextual menus are open.
This resolves #730.
---
 modules/libpref/init/all.js |  4 ++++
 widget/windows/nsWindow.cpp | 10 +++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index a3daae59e..1eee30210 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2209,6 +2209,10 @@ pref("ui.key.contentAccess", 5);
 pref("ui.key.menuAccessKeyFocuses", false); // overridden below
 pref("ui.key.saveLink.shift", true); // true = shift, false = meta
 
+// When true, overrides OS convention to lock content scrolling
+// if a contextual menu is open.
+pref("ui.menu.allow_content_scroll", false);
+
 // Disable page loading activity cursor by default.
 pref("ui.use_activity_cursor", false);
 
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index b2bb59bd3..5ce0117b0 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -366,6 +366,9 @@ static const int32_t kResizableBorderMinSize = 3;
 // Cached pointer events enabler value, True if pointer events are enabled.
 static bool gIsPointerEventsEnabled = false;
 
+// Cached scroll outside menu enabler value, True if scrolling is allowed.
+static bool gIsScrollingOutsideEnabled = false;
+
 // We should never really try to accelerate windows bigger than this. In some
 // cases this might lead to no D3D9 acceleration where we could have had it
 // but D3D9 does not reliably report when it supports bigger windows. 8192
@@ -665,6 +668,10 @@ nsWindow::nsWindow()
     Preferences::AddBoolVarCache(&gIsPointerEventsEnabled,
                                  "dom.w3c_pointer_events.enabled",
                                  gIsPointerEventsEnabled);
+    Preferences::AddBoolVarCache(&gIsScrollingOutsideEnabled,
+                                 "ui.menu.allow_content_scroll",
+                                 gIsScrollingOutsideEnabled);
+                                 
   } // !sInstanceCount
 
   mIdleService = nullptr;
@@ -7703,7 +7710,8 @@ nsWindow::DealWithPopups(HWND aWnd, UINT aMessage,
           break;
         }
       }
-      return consumeRollupEvent;
+      // Consume event if appropriate unless overridden.
+      return consumeRollupEvent && !gIsScrollingOutsideEnabled;
 
     case WM_ACTIVATEAPP:
       break;
-- 
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