firefox/mozilla-1493081.patch

diff --git a/view/nsView.cpp b/view/nsView.cpp
--- a/view/nsView.cpp
+++ b/view/nsView.cpp
@@ -320,7 +320,6 @@
   // Stash a copy of these and use them so we can handle this being deleted (say
   // from sync painting/flushing from Show/Move/Resize on the widget).
   LayoutDeviceIntRect newBounds;
-  RefPtr<nsDeviceContext> dx = mViewManager->GetDeviceContext();
 
   nsWindowType type = widget->WindowType();
 
@@ -360,7 +359,8 @@
   // because of the potential for device-pixel coordinate spaces for mixed
   // hidpi/lodpi screens to overlap each other and result in bad placement
   // (bug 814434).
-  DesktopToLayoutDeviceScale scale = dx->GetDesktopToDeviceScale();
+
+  DesktopToLayoutDeviceScale scale = widget->GetDesktopToDeviceScaleByScreen();
 
   DesktopRect deskRect = newBounds / scale;
   if (changedPos) {
diff --git a/widget/gtk/nsWindow.h b/widget/gtk/nsWindow.h
--- a/widget/gtk/nsWindow.h
+++ b/widget/gtk/nsWindow.h
@@ -125,6 +125,7 @@
     virtual float      GetDPI() override;
     virtual double     GetDefaultScaleInternal() override;
     mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() override;
+    mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScaleByScreen() override;
     virtual void       SetParent(nsIWidget* aNewParent) override;
     virtual void       SetModal(bool aModal) override;
     virtual bool       IsVisible() const override;
diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -57,6 +57,7 @@
 
 #if defined(MOZ_WAYLAND)
 #include <gdk/gdkwayland.h>
+#include "nsView.h"
 #endif
 
 #include "nsGkAtoms.h"
@@ -841,6 +842,37 @@
     return DesktopToLayoutDeviceScale(1.0);
 }
 
+DesktopToLayoutDeviceScale
+nsWindow::GetDesktopToDeviceScaleByScreen()
+{
+#ifdef MOZ_WAYLAND
+    GdkDisplay* gdkDisplay = gdk_display_get_default();
+    // In Wayland there's no way to get absolute position of the window and use it to
+    // determine the screen factor of the monitor on which the window is placed.
+    // The window is notified of the current scale factor but not at this point,
+    // so the GdkScaleFactor can return wrong value which can lead to wrong popup
+    // placement.
+    // We need to use parent's window scale factor for the new one.
+    if (GDK_IS_WAYLAND_DISPLAY(gdkDisplay)) {
+        nsView* view = nsView::GetViewFor(this);
+        if (view) {
+            nsView* parentView = view->GetParent();
+            if (parentView) {
+                nsIWidget* parentWidget = parentView->GetNearestWidget(nullptr);
+                if (parentWidget) {
+                    return DesktopToLayoutDeviceScale(parentWidget->RoundsWidgetCoordinatesTo());
+                } else {
+                    NS_WARNING("Widget has no parent");
+                }
+            }
+        } else {
+            NS_WARNING("Cannot find widget view");
+        }
+    }
+#endif
+    return nsBaseWidget::GetDesktopToDeviceScale();
+}
+
 void
 nsWindow::SetParent(nsIWidget *aNewParent)
 {
diff --git a/widget/nsBaseWidget.h b/widget/nsBaseWidget.h
--- a/widget/nsBaseWidget.h
+++ b/widget/nsBaseWidget.h
@@ -24,6 +24,8 @@
 #include "nsIWidgetListener.h"
 #include "nsPIDOMWindow.h"
 #include "nsWeakReference.h"
+#include "nsView.h"
+#include "nsViewManager.h"
 #include <algorithm>
 
 #if defined(XP_WIN)
@@ -238,6 +240,10 @@
   mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() override {
     return mozilla::DesktopToLayoutDeviceScale(1.0);
   }
+  mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScaleByScreen() override {
+    return (nsView::GetViewFor(this)->GetViewManager()->GetDeviceContext())->GetDesktopToDeviceScale();
+  }
+
   virtual void            ConstrainPosition(bool aAllowSlop,
                                             int32_t *aX,
                                             int32_t *aY) override {}
diff --git a/widget/nsIWidget.h b/widget/nsIWidget.h
--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -578,6 +578,13 @@
     virtual mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScale() = 0;
 
     /**
+     * Return the scaling factor between device pixels and the platform-
+     * dependent "desktop pixels" by looking up the screen by the position
+     * of the widget.
+     */
+    virtual mozilla::DesktopToLayoutDeviceScale GetDesktopToDeviceScaleByScreen() = 0;
+
+    /**
      * Return the default scale factor for the window. This is the
      * default number of device pixels per CSS pixel to use. This should
      * depend on OS/platform settings such as the Mac's "UI scale factor"