1 files changed, 25 insertions, 3 deletions
diff --git a/widget/cocoa/ScreenHelperCocoa.mm b/widget/cocoa/ScreenHelperCocoa.mm
index 57e1313320..2f1408e345 100644
--- a/widget/cocoa/ScreenHelperCocoa.mm
+++ b/widget/cocoa/ScreenHelperCocoa.mm
@@ -9,6 +9,7 @@
 #import <Cocoa/Cocoa.h>
 
 #include "mozilla/Logging.h"
+#include "nsCocoaFeatures.h"
 #include "nsCocoaUtils.h"
 #include "nsObjCExceptions.h"
 
@@ -109,6 +110,26 @@ static already_AddRefed<Screen> MakeScreen(NSScreen* aScreen) {
     pixelDepth = MAX_REPORTED_PIXEL_DEPTH;
   }
 
+  // What's the maximum color component value this screen can display? This
+  // is a reasonable stand-in for measuring peak brightness.
+  CGFloat componentValueMax =
+      aScreen.maximumPotentialExtendedDynamicRangeColorComponentValue;
+
+  // Should we treat this as HDR? Based on spec at
+  // https://drafts.csswg.org/mediaqueries-5/#dynamic-range, we'll consider it
+  // HDR if it has pixel depth greater than 24, and if has high peak brightness,
+  // which we measure by checking if it can represent component values greater
+  // than 1.0.
+  //
+  // Also, on HDR screens, users may want to force SDR by setting a different
+  // colorspace, for example by using the "Photography (P3 D65)" preset. In that
+  // case, componentValueMax will be 1.0 and we want to treat the display as
+  // SDR.
+  bool isHDR = pixelDepth > 24 && componentValueMax > 1.0;
+
+  // Double-check HDR against the platform capabilities.
+  isHDR &= nsCocoaFeatures::OnBigSurOrLater();
+
   float dpi = 96.0f;
   CGDirectDisplayID displayID =
       [[[aScreen deviceDescription] objectForKey:@"NSScreenNumber"] intValue];
@@ -125,9 +146,10 @@ static already_AddRefed<Screen> MakeScreen(NSScreen* aScreen) {
   // Getting the refresh rate is a little hard on OS X. We could use
   // CVDisplayLinkGetNominalOutputVideoRefreshPeriod, but that's a little
   // involved. Ideally we could query it from vsync. For now, we leave it out.
-  RefPtr<Screen> screen = new Screen(rect, availRect, pixelDepth, pixelDepth, 0,
-                                     contentsScaleFactor, defaultCssScaleFactor,
-                                     dpi, Screen::IsPseudoDisplay::No);
+  RefPtr<Screen> screen =
+      new Screen(rect, availRect, pixelDepth, pixelDepth, 0,
+                 contentsScaleFactor, defaultCssScaleFactor, dpi,
+                 Screen::IsPseudoDisplay::No, Screen::IsHDR(isHDR));
   return screen.forget();
 
   NS_OBJC_END_TRY_BLOCK_RETURN(nullptr);