diff --git a/gfx/src/nsDeviceContext.cpp b/gfx/src/nsDeviceContext.cpp
index 58d2d9aa96cd..89f9ecf48bbf 100644
--- a/gfx/src/nsDeviceContext.cpp
+++ b/gfx/src/nsDeviceContext.cpp
@@ -251,7 +251,7 @@ nsDeviceContext::IsPrinterSurface()
 }
 
 void
-nsDeviceContext::SetDPI()
+nsDeviceContext::SetDPI(double* aScale)
 {
     float dpi = -1.0f;
 
@@ -280,9 +280,21 @@ nsDeviceContext::SetDPI()
             dpi = 96.0f;
         }
 
-        CSSToLayoutDeviceScale scale = mWidget ? mWidget->GetDefaultScale()
-                                               : CSSToLayoutDeviceScale(1.0);
-        double devPixelsPerCSSPixel = scale.scale;
+        double devPixelsPerCSSPixel;
+        if (aScale && *aScale > 0.0) {
+            // if caller provided a scale, we just use it
+            devPixelsPerCSSPixel = *aScale;
+        } else {
+            // otherwise get from the widget, and return it in aScale for
+            // the caller to pass to child contexts if needed
+            CSSToLayoutDeviceScale scale =
+                mWidget ? mWidget->GetDefaultScale()
+                        : CSSToLayoutDeviceScale(1.0);
+            devPixelsPerCSSPixel = scale.scale;
+            if (aScale) {
+                *aScale = devPixelsPerCSSPixel;
+            }
+        }
 
         mAppUnitsPerDevPixelAtUnitFullZoom =
             std::max(1, NS_lround(AppUnitsPerCSSPixel() / devPixelsPerCSSPixel));
@@ -632,11 +644,12 @@ nsDeviceContext::CalcPrintingSize()
     return (mWidth > 0 && mHeight > 0);
 }
 
-bool nsDeviceContext::CheckDPIChange() {
+bool nsDeviceContext::CheckDPIChange(double* aScale)
+{
     int32_t oldDevPixels = mAppUnitsPerDevPixelAtUnitFullZoom;
     int32_t oldInches = mAppUnitsPerPhysicalInch;
 
-    SetDPI();
+    SetDPI(aScale);
 
     return oldDevPixels != mAppUnitsPerDevPixelAtUnitFullZoom ||
         oldInches != mAppUnitsPerPhysicalInch;
diff --git a/gfx/src/nsDeviceContext.h b/gfx/src/nsDeviceContext.h
index 887125734634..b50e8af14de9 100644
--- a/gfx/src/nsDeviceContext.h
+++ b/gfx/src/nsDeviceContext.h
@@ -219,12 +219,18 @@ public:
     nsresult EndPage();
 
     /**
-     * Check to see if the DPI has changed
+     * Check to see if the DPI has changed, or impose a new DPI scale value.
+     * @param  aScale - If non-null, the default (unzoomed) CSS to device pixel
+     *                  scale factor will be returned here; and if it is > 0.0
+     *                  on input, the given value will be used instead of
+     *                  getting it from the widget (if any). This is used to
+     *                  allow subdocument contexts to inherit the resolution
+     *                  setting of their parent.
      * @return whether there was actually a change in the DPI (whether
      *         AppUnitsPerDevPixel() or AppUnitsPerPhysicalInch()
      *         changed)
      */
-    bool CheckDPIChange();
+    bool CheckDPIChange(double* aScale = nullptr);
 
     /**
      * Set the full zoom factor: all lengths are multiplied by this factor
@@ -249,7 +255,7 @@ private:
     // Private destructor, to discourage deletion outside of Release():
     ~nsDeviceContext();
 
-    void SetDPI();
+    void SetDPI(double* aScale = nullptr);
     void ComputeClientRectUsingScreen(nsRect *outRect);
     void ComputeFullAreaUsingScreen(nsRect *outRect);
     void FindScreen(nsIScreen **outScreen);
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 690e34d54ffd..40198fd3b366 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -1693,7 +1693,7 @@ nsPresContext::UIResolutionChangedSync()
 {
   if (!mPendingUIResolutionChanged) {
     mPendingUIResolutionChanged = true;
-    UIResolutionChangedInternal();
+    UIResolutionChangedInternalScale(0.0);
   }
 }
 
@@ -1705,7 +1705,12 @@ nsPresContext::UIResolutionChangedSubdocumentCallback(nsIDocument* aDocument,
   if (shell) {
     nsPresContext* pc = shell->GetPresContext();
     if (pc) {
-      pc->UIResolutionChangedInternal();
+      // For subdocuments, we want to apply the parent's scale, because there
+      // are cases where the subdoc's device context is connected to a widget
+      // that has an out-of-date resolution (it's on a different screen, but
+      // currently hidden, and will not be updated until shown): bug 1249279.
+      double scale = *static_cast<double*>(aData);
+      pc->UIResolutionChangedInternalScale(scale);
     }
   }
   return true;
@@ -1730,10 +1735,16 @@ NotifyChildrenUIResolutionChanged(nsPIDOMWindowOuter* aWindow)
 
 void
 nsPresContext::UIResolutionChangedInternal()
+{
+  UIResolutionChangedInternalScale(0.0);
+}
+
+void
+nsPresContext::UIResolutionChangedInternalScale(double aScale)
 {
   mPendingUIResolutionChanged = false;
 
-  mDeviceContext->CheckDPIChange();
+  mDeviceContext->CheckDPIChange(&aScale);
   if (mCurAppUnitsPerDevPixel != AppUnitsPerDevPixel()) {
     AppUnitsPerDevPixelChanged();
   }
@@ -1744,7 +1755,7 @@ nsPresContext::UIResolutionChangedInternal()
   }
 
   mDocument->EnumerateSubDocuments(UIResolutionChangedSubdocumentCallback,
-                                   nullptr);
+                                   &aScale);
 }
 
 void
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 8144a7a3e4b9..32649974dee2 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -1102,8 +1102,17 @@ protected:
   friend class nsRunnableMethod<nsPresContext>;
   void ThemeChangedInternal();
   void SysColorChangedInternal();
+
+  // update device context's resolution from the widget
   void UIResolutionChangedInternal();
 
+  // if aScale > 0.0, use it as resolution scale factor to the device context
+  // (otherwise get it from the widget)
+  void UIResolutionChangedInternalScale(double aScale);
+
+  // aData here is a pointer to a double that holds the CSS to device-pixel
+  // scale factor from the parent, which will be applied to the subdocument's
+  // device context instead of retrieving a scale from the widget.
   static bool
   UIResolutionChangedSubdocumentCallback(nsIDocument* aDocument, void* aData);