Fix for [fe932e457]: Scaling on aqua gets unexpectedly reset.

Author: csaba

changes.md

@@ -18,6 +18,7 @@ to the userbase.
 # Bug fixes
  - [tk inactive returns a negative value on Windows for long inactivity](https://core.tcl-lang.org/tk/tktview/3cb7c4)
  - [nanosvg possible zero malloc on file error](https://github.com/memononen/nanosvg/pull/291)
+ - [Scaling on aqua gets unexpectedly reset](https://core.tcl-lang.org/tk/tktview/fe932e)
 
 Release Tk 9.0.4 arises from the check-in with tag `core-9-0-4`.
 

macosx/tkMacOSXXStubs.c

@@ -94,8 +94,10 @@ TkMacOSXDisplayChanged(
 	DefaultDepthOfScreen(screen) = NSBitsPerPixelFromDepth([s depth]);
 	WidthOfScreen(screen) = bounds.size.width;
 	HeightOfScreen(screen) = bounds.size.height;
-	WidthMMOfScreen(screen) = (bounds.size.width * 381 + 720) / 1440; /* = 3/4 * 254/720 */
-	HeightMMOfScreen(screen) = (bounds.size.height * 381 + 720) / 1440; /* = 3/4 * 254/720 */
+	if (WidthMMOfScreen(screen) == 0) {
+	    WidthMMOfScreen(screen) = (bounds.size.width * 381 + 720) / 1440; /* = 3/4 * 254/720 */
+	    HeightMMOfScreen(screen) = (bounds.size.height * 381 + 720) / 1440; /* = 3/4 * 254/720 */
+	}
 
 	for (s in nsScreens) {
 	    maxBounds = NSUnionRect(maxBounds, [s visibleFrame]);