diff --git a/widget/windows/nsScreenManagerWin.cpp b/widget/windows/nsScreenManagerWin.cpp
index 67bd7a03527d..3e2319ff319f 100644
--- a/widget/windows/nsScreenManagerWin.cpp
+++ b/widget/windows/nsScreenManagerWin.cpp
@@ -5,6 +5,7 @@
 
 #include "nsScreenManagerWin.h"
 #include "nsScreenWin.h"
+#include "gfxWindowsPlatform.h"
 
 
 BOOL CALLBACK CountMonitors ( HMONITOR, HDC, LPRECT, LPARAM ioCount ) ;
@@ -64,7 +65,7 @@ nsScreenManagerWin :: CreateNewScreenObject ( HMONITOR inScreen )
 // Returns the screen that contains the rectangle. If the rect overlaps
 // multiple screens, it picks the screen with the greatest area of intersection.
 //
-// The coordinates are in pixels (not twips) and in screen coordinates.
+// The coordinates are in pixels (not twips) and in logical screen coordinates.
 //
 NS_IMETHODIMP
 nsScreenManagerWin :: ScreenForRect ( int32_t inLeft, int32_t inTop, int32_t inWidth, int32_t inHeight,
@@ -76,7 +77,14 @@ nsScreenManagerWin :: ScreenForRect ( int32_t inLeft, int32_t inTop, int32_t inW
     return NS_OK;
   }
 
-  RECT globalWindowBounds = { inLeft, inTop, inLeft + inWidth, inTop + inHeight };
+  // convert coordinates from logical to device pixels for MonitorFromRect
+  FLOAT dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+  RECT globalWindowBounds = {
+    NSToIntRound(dpiScale * inLeft),
+    NSToIntRound(dpiScale * inTop),
+    NSToIntRound(dpiScale * (inLeft + inWidth)),
+    NSToIntRound(dpiScale * (inTop + inHeight))
+  };
 
   HMONITOR genScreen = ::MonitorFromRect( &globalWindowBounds, MONITOR_DEFAULTTOPRIMARY );
 
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index 132d7d21e8ab..09d64b8f5678 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -1624,12 +1624,19 @@ NS_IMETHODIMP nsWindow::SetSizeMode(int32_t aMode) {
 }
 
 // Constrain a potential move to fit onscreen
+// Position (aX, aY) is specified in Windows screen (logical) pixels
 NS_METHOD nsWindow::ConstrainPosition(bool aAllowSlop,
                                       int32_t *aX, int32_t *aY)
 {
   if (!mIsTopWidgetWindow) // only a problem for top-level windows
     return NS_OK;
 
+  float dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+
+  // we need to use the window size in logical screen pixels
+  int32_t logWidth = std::max<int32_t>(NSToIntRound(mBounds.width / dpiScale), 1);
+  int32_t logHeight = std::max<int32_t>(NSToIntRound(mBounds.height / dpiScale), 1);
+
   bool doConstrain = false; // whether we have enough info to do anything
 
   /* get our playing field. use the current screen, or failing that
@@ -1641,23 +1648,20 @@ NS_METHOD nsWindow::ConstrainPosition(bool aAllowSlop,
     nsCOMPtr<nsIScreen> screen;
     int32_t left, top, width, height;
 
-    // zero size rects confuse the screen manager
-    width = mBounds.width > 0 ? mBounds.width : 1;
-    height = mBounds.height > 0 ? mBounds.height : 1;
-    screenmgr->ScreenForRect(*aX, *aY, width, height,
+    screenmgr->ScreenForRect(*aX, *aY, logWidth, logHeight,
                              getter_AddRefs(screen));
     if (screen) {
       if (mSizeMode != nsSizeMode_Fullscreen) {
         // For normalized windows, use the desktop work area.
-        screen->GetAvailRect(&left, &top, &width, &height);
+        screen->GetAvailRectDisplayPix(&left, &top, &width, &height);
       } else {
         // For full screen windows, use the desktop.
-        screen->GetRect(&left, &top, &width, &height);
+        screen->GetRectDisplayPix(&left, &top, &width, &height);
       }
       screenRect.left = left;
-      screenRect.right = left+width;
+      screenRect.right = left + width;
       screenRect.top = top;
-      screenRect.bottom = top+height;
+      screenRect.bottom = top + height;
       doConstrain = true;
     }
   } else {
@@ -1680,13 +1684,13 @@ NS_METHOD nsWindow::ConstrainPosition(bool aAllowSlop,
   }
 
   if (aAllowSlop) {
-    if (*aX < screenRect.left - mBounds.width + kWindowPositionSlop)
-      *aX = screenRect.left - mBounds.width + kWindowPositionSlop;
+    if (*aX < screenRect.left - logWidth + kWindowPositionSlop)
+      *aX = screenRect.left - logWidth + kWindowPositionSlop;
     else if (*aX >= screenRect.right - kWindowPositionSlop)
       *aX = screenRect.right - kWindowPositionSlop;
 
-    if (*aY < screenRect.top - mBounds.height + kWindowPositionSlop)
-      *aY = screenRect.top - mBounds.height + kWindowPositionSlop;
+    if (*aY < screenRect.top - logHeight + kWindowPositionSlop)
+      *aY = screenRect.top - logHeight + kWindowPositionSlop;
     else if (*aY >= screenRect.bottom - kWindowPositionSlop)
       *aY = screenRect.bottom - kWindowPositionSlop;
 
@@ -1694,13 +1698,13 @@ NS_METHOD nsWindow::ConstrainPosition(bool aAllowSlop,
 
     if (*aX < screenRect.left)
       *aX = screenRect.left;
-    else if (*aX >= screenRect.right - mBounds.width)
-      *aX = screenRect.right - mBounds.width;
+    else if (*aX >= screenRect.right - logWidth)
+      *aX = screenRect.right - logWidth;
 
     if (*aY < screenRect.top)
       *aY = screenRect.top;
-    else if (*aY >= screenRect.bottom - mBounds.height)
-      *aY = screenRect.bottom - mBounds.height;
+    else if (*aY >= screenRect.bottom - logHeight)
+      *aY = screenRect.bottom - logHeight;
   }
 
   return NS_OK;