app: added support for fullscreen mode

The option field WindowMode allows changing the window mode of an application in either Windowed or Fullscreen.
Only macOS, Windows and X11 platforms are currently supported.

Updates gio#89.

Signed-off-by: pierre <pierre.curto@gmail.com>

app/internal/wm/os_windows.go

@@ -52,6 +52,9 @@ type window struct {
 	cursorIn bool
 	cursor   syscall.Handle
 
+	// placement saves the previous window position when in full screen mode.
+	placement *windows.WindowPlacement
+
 	mu        sync.Mutex
 	animating bool
 
@@ -119,6 +122,7 @@ func NewWindow(window Callbacks, opts *Options) error {
 		// Since the window class for the cursor is null,
 		// set it here to show the cursor.
 		w.SetCursor(pointer.CursorDefault)
+		w.SetWindowMode(opts.WindowMode)
 		if err := w.loop(); err != nil {
 			panic(err)
 		}
@@ -524,6 +528,38 @@ func (w *window) readClipboard() error {
 	return nil
 }
 
+func (w *window) SetWindowMode(mode WindowMode) {
+	// https://devblogs.microsoft.com/oldnewthing/20100412-00/?p=14353
+	switch mode {
+	case Windowed:
+		if w.placement == nil {
+			return
+		}
+		windows.SetWindowPlacement(w.hwnd, w.placement)
+		w.placement = nil
+		style := windows.GetWindowLong(w.hwnd)
+		windows.SetWindowLong(w.hwnd, windows.GWL_STYLE, style|windows.WS_OVERLAPPEDWINDOW)
+		windows.SetWindowPos(w.hwnd, windows.HWND_TOPMOST,
+			0, 0, 0, 0,
+			windows.SWP_NOOWNERZORDER|windows.SWP_FRAMECHANGED,
+		)
+	case Fullscreen:
+		if w.placement != nil {
+			return
+		}
+		w.placement = windows.GetWindowPlacement(w.hwnd)
+		style := windows.GetWindowLong(w.hwnd)
+		windows.SetWindowLong(w.hwnd, windows.GWL_STYLE, style&^windows.WS_OVERLAPPEDWINDOW)
+		mi := windows.GetMonitorInfo(w.hwnd)
+		windows.SetWindowPos(w.hwnd, 0,
+			mi.Monitor.Left, mi.Monitor.Top,
+			mi.Monitor.Right-mi.Monitor.Left,
+			mi.Monitor.Bottom-mi.Monitor.Top,
+			windows.SWP_NOOWNERZORDER|windows.SWP_FRAMECHANGED,
+		)
+	}
+}
+
 func (w *window) WriteClipboard(s string) {
 	w.writeClipboard(s)
 }