--- a/widget/tests/test_scrollbar_colors.html
+++ b/widget/tests/test_scrollbar_colors.html
@@ -38,17 +38,17 @@ function countPixels(canvas) {
 }
 
 const WIN_REFERENCES = [
   // Yellow background
   ["255,255,0", 6889],
   // Blue scrollbar face
   ["0,0,255", 540],
   // Cyan scrollbar track
-  ["0,255,255", 2198],
+  ["0,255,255", 2487],
 ];
 
 SimpleTest.waitForExplicitFinish();
 SpecialPowers.pushPrefEnv({
   "set": [["layout.css.scrollbar-colors.enabled", true]],
 }, function() {
   document.querySelector('#style').textContent = `
     .outer {

--- a/widget/windows/nsNativeThemeWin.cpp
+++ b/widget/windows/nsNativeThemeWin.cpp
@@ -758,16 +758,17 @@ mozilla::Maybe<nsUXThemeClass> nsNativeT
     case NS_THEME_SCROLLBAR_VERTICAL:
     case NS_THEME_SCROLLBAR_HORIZONTAL:
     case NS_THEME_SCROLLBARBUTTON_UP:
     case NS_THEME_SCROLLBARBUTTON_DOWN:
     case NS_THEME_SCROLLBARBUTTON_LEFT:
     case NS_THEME_SCROLLBARBUTTON_RIGHT:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:
+    case NS_THEME_SCROLLCORNER:
       return Some(eUXScrollbar);
     case NS_THEME_RANGE:
     case NS_THEME_RANGE_THUMB:
     case NS_THEME_SCALE_HORIZONTAL:
     case NS_THEME_SCALE_VERTICAL:
     case NS_THEME_SCALETHUMB_HORIZONTAL:
     case NS_THEME_SCALETHUMB_VERTICAL:
       return Some(eUXTrackbar);
@@ -1164,17 +1165,18 @@ nsNativeThemeWin::GetThemePartAndState(n
       return NS_OK;    
     }
     case NS_THEME_TOOLBOX:
     case NS_THEME_WIN_MEDIA_TOOLBOX:
     case NS_THEME_WIN_COMMUNICATIONS_TOOLBOX:
     case NS_THEME_WIN_BROWSERTABBAR_TOOLBOX:
     case NS_THEME_STATUSBAR:
     case NS_THEME_SCROLLBAR:
-    case NS_THEME_SCROLLBAR_SMALL: {
+    case NS_THEME_SCROLLBAR_SMALL:
+    case NS_THEME_SCROLLCORNER: {
       aState = 0;
       aPart = RP_BACKGROUND;
       return NS_OK;
     }
     case NS_THEME_TOOLBAR: {
       // Use -1 to indicate we don't wish to have the theme background drawn
       // for this item. We will pass any nessessary information via aState,
       // and will render the item using separate code.
@@ -1533,16 +1535,17 @@ IsWidgetScrollbarPart(uint8_t aWidgetTyp
     case NS_THEME_SCROLLBAR_VERTICAL:
     case NS_THEME_SCROLLBAR_HORIZONTAL:
     case NS_THEME_SCROLLBARBUTTON_UP:
     case NS_THEME_SCROLLBARBUTTON_DOWN:
     case NS_THEME_SCROLLBARBUTTON_LEFT:
     case NS_THEME_SCROLLBARBUTTON_RIGHT:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:
+    case NS_THEME_SCROLLCORNER:
       return true;
     default:
       return false;
   }
 }
 
 NS_IMETHODIMP
 nsNativeThemeWin::DrawWidgetBackground(gfxContext* aContext,
@@ -2051,16 +2054,17 @@ nsNativeThemeWin::GetWidgetBorder(nsDevi
       aWidgetType == NS_THEME_TOOLBOX || 
       aWidgetType == NS_THEME_WIN_MEDIA_TOOLBOX ||
       aWidgetType == NS_THEME_WIN_COMMUNICATIONS_TOOLBOX ||
       aWidgetType == NS_THEME_WIN_BROWSERTABBAR_TOOLBOX ||
       aWidgetType == NS_THEME_STATUSBAR || 
       aWidgetType == NS_THEME_RESIZER || aWidgetType == NS_THEME_TABPANEL ||
       aWidgetType == NS_THEME_SCROLLBAR_HORIZONTAL ||
       aWidgetType == NS_THEME_SCROLLBAR_VERTICAL ||
+      aWidgetType == NS_THEME_SCROLLCORNER ||
       aWidgetType == NS_THEME_MENUITEM || aWidgetType == NS_THEME_CHECKMENUITEM ||
       aWidgetType == NS_THEME_RADIOMENUITEM || aWidgetType == NS_THEME_MENUPOPUP ||
       aWidgetType == NS_THEME_MENUIMAGE || aWidgetType == NS_THEME_MENUITEMTEXT ||
       aWidgetType == NS_THEME_SEPARATOR ||
       aWidgetType == NS_THEME_WINDOW_TITLEBAR ||
       aWidgetType == NS_THEME_WINDOW_TITLEBAR_MAXIMIZED ||
       aWidgetType == NS_THEME_WIN_GLASS || aWidgetType == NS_THEME_WIN_BORDERLESS_GLASS)
     return result; // Don't worry about it.
@@ -2419,16 +2423,17 @@ nsNativeThemeWin::GetMinimumWidgetSize(n
         aResult->width = 20;
         aResult->height = 12;
       }
       ScaleForFrameDPI(aResult, aFrame);
       return rv;
     }
 
     case NS_THEME_SCROLLBAR:
+    case NS_THEME_SCROLLCORNER:
     {
       if (nsLookAndFeel::GetInt(
             nsLookAndFeel::eIntID_UseOverlayScrollbars) != 0) {
         aResult->SizeTo(::GetSystemMetrics(SM_CXHSCROLL),
                         ::GetSystemMetrics(SM_CYVSCROLL));
         ScaleForFrameDPI(aResult, aFrame);
         return rv;
       }
@@ -2701,16 +2706,17 @@ nsNativeThemeWin::ThemeGeometryTypeForWi
 }
 
 nsITheme::Transparency
 nsNativeThemeWin::GetWidgetTransparency(nsIFrame* aFrame, uint8_t aWidgetType)
 {
   switch (aWidgetType) {
   case NS_THEME_SCROLLBAR_SMALL:
   case NS_THEME_SCROLLBAR:
+  case NS_THEME_SCROLLCORNER:
   case NS_THEME_STATUSBAR:
     // Knowing that scrollbars and statusbars are opaque improves
     // performance, because we create layers for them. This better be
     // true across all Windows themes! If it's not true, we should
     // paint an opaque background for them to make it true!
     return eOpaque;
   case NS_THEME_WIN_GLASS:
   case NS_THEME_WIN_BORDERLESS_GLASS:
@@ -2783,16 +2789,17 @@ nsNativeThemeWin::ClassicThemeSupportsWi
     case NS_THEME_SCROLLBARBUTTON_DOWN:
     case NS_THEME_SCROLLBARBUTTON_LEFT:
     case NS_THEME_SCROLLBARBUTTON_RIGHT:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:
     case NS_THEME_SCROLLBAR_VERTICAL:
     case NS_THEME_SCROLLBAR_HORIZONTAL:
     case NS_THEME_SCROLLBAR_NON_DISAPPEARING:
+    case NS_THEME_SCROLLCORNER:
     case NS_THEME_SCALE_HORIZONTAL:
     case NS_THEME_SCALE_VERTICAL:
     case NS_THEME_SCALETHUMB_HORIZONTAL:
     case NS_THEME_SCALETHUMB_VERTICAL:
     case NS_THEME_MENULIST_BUTTON:
     case NS_THEME_INNER_SPIN_BUTTON:
     case NS_THEME_SPINNER_UPBUTTON:
     case NS_THEME_SPINNER_DOWNBUTTON:
@@ -3256,16 +3263,17 @@ nsresult nsNativeThemeWin::ClassicGetThe
     case NS_THEME_MENULIST:
     case NS_THEME_MENULIST_TEXTFIELD:
     case NS_THEME_RANGE:
     case NS_THEME_RANGE_THUMB:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:     
     case NS_THEME_SCROLLBAR_VERTICAL:
     case NS_THEME_SCROLLBAR_HORIZONTAL:      
+    case NS_THEME_SCROLLCORNER:
     case NS_THEME_SCALE_HORIZONTAL:
     case NS_THEME_SCALE_VERTICAL:
     case NS_THEME_SCALETHUMB_HORIZONTAL:
     case NS_THEME_SCALETHUMB_VERTICAL:
     case NS_THEME_STATUSBAR:
     case NS_THEME_STATUSBARPANEL:
     case NS_THEME_RESIZERPANEL:
     case NS_THEME_PROGRESSCHUNK:
@@ -3777,16 +3785,19 @@ RENDER_AGAIN:
         DrawCheckedRect(hdc, widgetRect, COLOR_3DHILIGHT, COLOR_3DFACE,
                         (HBRUSH) COLOR_SCROLLBAR+1);
       }
       // XXX should invert the part of the track being clicked here
       // but the track is never :active
 
       break;
     }
+    case NS_THEME_SCROLLCORNER: {
+      ::FillRect(hdc, &widgetRect, (HBRUSH) (COLOR_SCROLLBAR + 1));
+    }
     // Draw scale track background
     case NS_THEME_RANGE:
     case NS_THEME_SCALE_VERTICAL:
     case NS_THEME_SCALE_HORIZONTAL: { 
       const int32_t trackWidth = 4;
       // When rounding is necessary, we round the position of the track
       // away from the chevron of the thumb to make it look better.
       if (aWidgetType == NS_THEME_SCALE_HORIZONTAL ||
@@ -4058,16 +4069,17 @@ nsNativeThemeWin::GetWidgetNativeDrawing
     case NS_THEME_SCROLLBARBUTTON_UP:
     case NS_THEME_SCROLLBARBUTTON_DOWN:
     case NS_THEME_SCROLLBARBUTTON_LEFT:
     case NS_THEME_SCROLLBARBUTTON_RIGHT:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:
     case NS_THEME_SCROLLBAR_VERTICAL:
     case NS_THEME_SCROLLBAR_HORIZONTAL:
+    case NS_THEME_SCROLLCORNER:
     case NS_THEME_SCALE_HORIZONTAL:
     case NS_THEME_SCALE_VERTICAL:
     case NS_THEME_SCALETHUMB_HORIZONTAL:
     case NS_THEME_SCALETHUMB_VERTICAL:
     case NS_THEME_INNER_SPIN_BUTTON:
     case NS_THEME_SPINNER_UPBUTTON:
     case NS_THEME_SPINNER_DOWNBUTTON:
     case NS_THEME_LISTBOX: