Bug 42528: Don't leak system scrollbar size on windows.

widget/ScrollbarDrawingWin.cpp

@@ -11,6 +11,7 @@
 #include "nsLayoutUtils.h"
 #include "Theme.h"
 #include "nsNativeTheme.h"
+#include "nsContentUtils.h"
 
 namespace mozilla::widget {
 
@@ -164,7 +165,10 @@ void ScrollbarDrawingWin::RecomputeScrollbarParams() {
   }
   ConfigureScrollbarSize(defaultSize);
 
-  if (StaticPrefs::widget_non_native_theme_win_scrollbar_use_system_size()) {
+  // Do not leak system size when using ResistFingerprinting.
+  if (!nsContentUtils::ShouldResistFingerprinting("No context available",
+                                                  RFPTarget::CSSResolution) &&
+      StaticPrefs::widget_non_native_theme_win_scrollbar_use_system_size()) {
     ConfigureScrollbarSize(LookAndFeel::GetInt(
         LookAndFeel::IntID::SystemScrollbarSize, defaultSize));
   }

widget/ScrollbarDrawingWin11.cpp

@@ -11,6 +11,7 @@
 #include "nsLayoutUtils.h"
 #include "Theme.h"
 #include "nsNativeTheme.h"
+#include "nsContentUtils.h"
 
 using mozilla::gfx::sRGBColor;
 
@@ -352,6 +353,11 @@ bool ScrollbarDrawingWin11::PaintScrollbarThumb(
 
 void ScrollbarDrawingWin11::RecomputeScrollbarParams() {
   ScrollbarDrawingWin::RecomputeScrollbarParams();
+  if (nsContentUtils::ShouldResistFingerprinting("No context available",
+                                                 RFPTarget::CSSResolution)) {
+    // Do not distinguish sizes between windows 10 and 11.
+    return;
+  }
   // TODO(emilio): Maybe make this configurable? Though this doesn't respect
   // classic Windows registry settings, and cocoa overlay scrollbars also don't
   // respect the override it seems, so this should be fine.