Merge pull request #42478 from bruvzg/macos_dpi_32

[3.2] Fix `get_screen_dpi` on macOS.

doc/classes/OS.xml

@@ -361,7 +361,8 @@
 			</argument>
 			<description>
 				Returns the dots per inch density of the specified screen. If [code]screen[/code] is [/code]-1[/code] (the default value), the current screen will be used.
-				On Android devices, the actual screen densities are grouped into six generalized densities:
+				[b]Note:[/b] On macOS, returned value is inaccurate if fractional display scaling mode is used.
+				[b]Note:[/b] On Android devices, the actual screen densities are grouped into six generalized densities:
 				[codeblock]
 				   ldpi - 120 dpi
 				   mdpi - 160 dpi

platform/osx/os_osx.mm

@@ -2471,8 +2471,15 @@ int OS_OSX::get_screen_dpi(int p_screen) const {
 	NSArray *screenArray = [NSScreen screens];
 	if ((NSUInteger)p_screen < [screenArray count]) {
 		NSDictionary *description = [[screenArray objectAtIndex:p_screen] deviceDescription];
-		NSSize displayDPI = [[description objectForKey:NSDeviceResolution] sizeValue];
-		return (displayDPI.width + displayDPI.height) / 2;
+
+		const NSSize displayPixelSize = [[description objectForKey:NSDeviceSize] sizeValue];
+		const CGSize displayPhysicalSize = CGDisplayScreenSize([[description objectForKey:@"NSScreenNumber"] unsignedIntValue]);
+		float scale = [[screenArray objectAtIndex:p_screen] backingScaleFactor];
+
+		float den2 = (displayPhysicalSize.width / 25.4f) * (displayPhysicalSize.width / 25.4f) + (displayPhysicalSize.height / 25.4f) * (displayPhysicalSize.height / 25.4f);
+		if (den2 > 0.0f) {
+			return ceil(sqrt(displayPixelSize.width * displayPixelSize.width + displayPixelSize.height * displayPixelSize.height) / sqrt(den2) * scale);
+		}
 	}
 
 	return 72;