--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -7874,17 +7874,21 @@ nsIDocument::AdoptNode(nsINode& aAdopted
 }
 
 nsViewportInfo
 nsDocument::GetViewportInfo(const ScreenIntSize& aDisplaySize)
 {
   // Compute the CSS-to-LayoutDevice pixel scale as the product of the
   // widget scale and the full zoom.
   nsPresContext* context = mPresShell->GetPresContext();
-  float fullZoom = context ? context->GetFullZoom() : 1.0;
+  // When querying the full zoom, get it from the device context rather than
+  // directly from the pres context, because the device context's value can
+  // include an adjustment necessay to keep the number of app units per device
+  // pixel an integer, and we want the adjusted value.
+  float fullZoom = context ? context->DeviceContext()->GetFullZoom() : 1.0;
   fullZoom = (fullZoom == 0.0) ? 1.0 : fullZoom;
   nsIWidget *widget = nsContentUtils::WidgetForDocument(this);
   float widgetScale = widget ? widget->GetDefaultScale().scale : 1.0f;
   CSSToLayoutDeviceScale layoutDeviceScale(widgetScale * fullZoom);
 
   CSSToScreenScale defaultScale = layoutDeviceScale
                                 * LayoutDeviceToScreenScale(1.0);
   // Get requested Desktopmode