Bring back borderless mode for macOS and Linux

osu.Framework/Platform/SDL3/SDL3Extensions.cs

@@ -860,16 +860,17 @@ public static SDL_Scancode ToScancode(this InputKey inputKey)
             }
         }
 
-        public static WindowState ToWindowState(this SDL_WindowFlags windowFlags)
+        public static WindowState ToWindowState(this SDL_WindowFlags windowFlags, bool isFullscreenBorderless)
         {
+            // for windows
             if (windowFlags.HasFlagFast(SDL_WindowFlags.SDL_WINDOW_BORDERLESS))
                 return WindowState.FullscreenBorderless;
 
             if (windowFlags.HasFlagFast(SDL_WindowFlags.SDL_WINDOW_MINIMIZED))
                 return WindowState.Minimised;
 
             if (windowFlags.HasFlagFast(SDL_WindowFlags.SDL_WINDOW_FULLSCREEN))
-                return WindowState.Fullscreen;
+                return isFullscreenBorderless ? WindowState.FullscreenBorderless : WindowState.Fullscreen;
 
             if (windowFlags.HasFlagFast(SDL_WindowFlags.SDL_WINDOW_MAXIMIZED))
                 return WindowState.Maximised;

osu.Framework/Platform/SDL3/SDL3Window.cs

@@ -316,7 +316,7 @@ protected void HandleEventFromWatch(SDL_Event evt)
                 case SDL_EventType.SDL_EVENT_WINDOW_RESIZED:
                     // polling via SDL_PollEvent blocks on resizes (https://stackoverflow.com/a/50858339)
                     if (!updatingWindowStateAndSize)
-                        fetchWindowSize();
+                        fetchWindowSize(storeToConfig: false);
 
                     break;
             }

osu.Framework/Platform/SDL3/SDL3Window_Windowing.cs

@@ -149,10 +149,7 @@ public virtual IEnumerable<WindowMode> SupportedWindowModes
                 if (RuntimeInfo.IsMobile)
                     return new[] { Configuration.WindowMode.Fullscreen };
 
-                if (RuntimeInfo.OS == RuntimeInfo.Platform.Windows)
-                    return Enum.GetValues<WindowMode>();
-
-                return new[] { Configuration.WindowMode.Windowed, Configuration.WindowMode.Fullscreen };
+                return Enum.GetValues<WindowMode>();
             }
         }
 
@@ -447,7 +444,7 @@ private unsafe Rectangle windowDisplayBounds
         /// Updates <see cref="Size"/> and <see cref="Scale"/> according to SDL state.
         /// </summary>
         /// <returns>Whether the window size has been changed after updating.</returns>
-        private unsafe void fetchWindowSize()
+        private unsafe void fetchWindowSize(bool storeToConfig = true)
         {
             int w, h;
             SDL_GetWindowSize(SDLWindowHandle, &w, &h);
@@ -462,7 +459,8 @@ private unsafe void fetchWindowSize()
             Scale = (float)drawableW / w;
             Size = new Size(w, h);
 
-            storeWindowSizeToConfig();
+            if (storeToConfig)
+                storeWindowSizeToConfig();
         }
 
         #region SDL Event Handling
@@ -583,7 +581,7 @@ private unsafe void updateAndFetchWindowSpecifics()
             }
             else
             {
-                windowState = SDL_GetWindowFlags(SDLWindowHandle).ToWindowState();
+                windowState = SDL_GetWindowFlags(SDLWindowHandle).ToWindowState(SDL_GetWindowFullscreenMode(SDLWindowHandle) == null);
             }
 
             if (windowState != stateBefore)
@@ -805,7 +803,16 @@ private void storeWindowSizeToConfig()
         /// <returns>
         /// The size of the borderless window's draw area.
         /// </returns>
-        protected virtual Size SetBorderless(Display display) => throw new PlatformNotSupportedException();
+        protected virtual unsafe Size SetBorderless(Display display)
+        {
+            ensureWindowOnDisplay(display);
+
+            // this is a generally sane method of handling borderless, and works well on macOS and linux.
+            SDL_SetWindowFullscreenMode(SDLWindowHandle, null);
+            SDL_SetWindowFullscreen(SDLWindowHandle, true);
+
+            return display.Bounds.Size;
+        }
 
         #endregion