ui/cocoa: Fix aspect ratio

[NSWindow setContentAspectRatio:] does not trigger window resize itself,
so the wrong aspect ratio will persist if nothing resizes the window.
Call [NSWindow setContentSize:] in such a case.

Fixes: 91aa508d02 ("ui/cocoa: Let the platform toggle fullscreen")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20240323-fixes-v2-1-18651a2b0394@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

ui/cocoa.m

@@ -519,6 +519,43 @@ static CGEventRef handleTapEvent(CGEventTapProxy proxy, CGEventType type, CGEven
     }
 }
 
+- (NSSize)fixAspectRatio:(NSSize)max
+{
+    NSSize scaled;
+    NSSize fixed;
+
+    scaled.width = screen.width * max.height;
+    scaled.height = screen.height * max.width;
+
+    /*
+     * Here screen is our guest's output size, and max is the size of the
+     * largest possible area of the screen we can display on.
+     * We want to scale up (screen.width x screen.height) by either:
+     *   1) max.height / screen.height
+     *   2) max.width / screen.width
+     * With the first scale factor the scale will result in an output height of
+     * max.height (i.e. we will fill the whole height of the available screen
+     * space and have black bars left and right) and with the second scale
+     * factor the scaling will result in an output width of max.width (i.e. we
+     * fill the whole width of the available screen space and have black bars
+     * top and bottom). We need to pick whichever keeps the whole of the guest
+     * output on the screen, which is to say the smaller of the two scale
+     * factors.
+     * To avoid doing more division than strictly necessary, instead of directly
+     * comparing scale factors 1 and 2 we instead calculate and compare those
+     * two scale factors multiplied by (screen.height * screen.width).
+     */
+    if (scaled.width < scaled.height) {
+        fixed.width = scaled.width / screen.height;
+        fixed.height = max.height;
+    } else {
+        fixed.width = max.width;
+        fixed.height = scaled.height / screen.width;
+    }
+
+    return fixed;
+}
+
 - (NSSize) screenSafeAreaSize
 {
     NSSize size = [[[self window] screen] frame].size;
@@ -536,8 +573,10 @@ static CGEventRef handleTapEvent(CGEventTapProxy proxy, CGEventType type, CGEven
         [[self window] setContentSize:NSMakeSize(screen.width, screen.height)];
         [[self window] center];
     } else if ([[self window] styleMask] & NSWindowStyleMaskFullScreen) {
-        [[self window] setContentSize:[self screenSafeAreaSize]];
+        [[self window] setContentSize:[self fixAspectRatio:[self screenSafeAreaSize]]];
         [[self window] center];
+    } else {
+        [[self window] setContentSize:[self fixAspectRatio:[self frame].size]];
     }
 }