Full-screen: Retain uniform x and y scaling, unless user specifies a full-screen resolution (#1121)

source/Windows/AppleWin.cpp

@@ -64,6 +64,8 @@ static bool g_bSysClkOK = false;
 
 bool g_bRestartFullScreen = false;
 
+static bool g_fullScreenResolutionChangedByUser = false;
+
 //===========================================================================
 
 bool GetLoadedSaveStateFlag(void)
@@ -86,6 +88,11 @@ bool GetHookAltGrControl(void)
 	return g_bHookAltGrControl;
 }
 
+bool GetFullScreenResolutionChangedByUser(void)
+{
+	return g_fullScreenResolutionChangedByUser;
+}
+
 static void ResetToLogoMode(void)
 {
 	g_nAppMode = MODE_LOGO;
@@ -809,6 +816,9 @@ static void RepeatInitialization(void)
 				LogFileOutput("Best resolution for -fs-width/height=x switch(es): Width=%d, Height=%d\n", bestWidth, bestHeight);
 			else
 				LogFileOutput("Failed to set parameter for -fs-width/height=x switch(es)\n");
+
+			if (res)
+				g_fullScreenResolutionChangedByUser = true;
 		}
 
 		// Pre: may need g_hFrameWindow for MessageBox errors

source/Windows/AppleWin.h

@@ -8,5 +8,6 @@ void SingleStep(bool bReinit);
 bool GetLoadedSaveStateFlag(void);
 bool GetHookAltTab(void);
 bool GetHookAltGrControl(void);
+bool GetFullScreenResolutionChangedByUser(void);
 
 extern bool g_bRestartFullScreen;

source/Windows/WinFrame.cpp

@@ -2149,6 +2149,13 @@ void Win32Frame::SetFullScreenMode(void)
 	scalex = width  / GetVideo().GetFrameBufferBorderlessWidth();
 	scaley = height / GetVideo().GetFrameBufferBorderlessHeight();
 
+	// Retain aspect ratio if user hasn't changed full-screen resolution (GH#1121)
+	if (!GetFullScreenResolutionChangedByUser())
+	{
+		const int minimumScale = (scalex <= scaley) ? scalex : scaley;
+		scalex = scaley = minimumScale;
+	}
+
 	// NB. Separate x,y scaling is OK in full-screen mode
 	// . eg. SHR 640x400 (scalex=2, scaley=3) => 1280x1200, which roughly gives a 4:3 aspect ratio for a resolution of 1600x1200
 	g_win_fullscreen_offsetx = ((int)width - (int)(scalex * GetVideo().GetFrameBufferBorderlessWidth())) / 2;