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_x11.go

@@ -71,6 +71,12 @@ type x11Window struct {
 		atom C.Atom
 		// "GTK_TEXT_BUFFER_CONTENTS"
 		gtk_text_buffer_contents C.Atom
+		// "_NET_WM_NAME"
+		wmName C.Atom
+		// "_NET_WM_STATE"
+		wmState C.Atom
+		// _NET_WM_STATE_FULLSCREEN"
+		wmStateFullscreen C.Atom
 	}
 	stage  system.Stage
 	cfg    unit.Metric
@@ -141,6 +147,18 @@ func (w *x11Window) SetCursor(name pointer.CursorName) {
 	C.XDefineCursor(w.x, w.xw, c)
 }
 
+func (w *x11Window) SetWindowMode(mode WindowMode) {
+	switch mode {
+	case Windowed:
+		C.XDeleteProperty(w.x, w.xw, w.atoms.wmStateFullscreen)
+	case Fullscreen:
+		C.XChangeProperty(w.x, w.xw, w.atoms.wmState, C.XA_ATOM,
+			32, C.PropModeReplace,
+			(*C.uchar)(unsafe.Pointer(&w.atoms.wmStateFullscreen)), 1,
+		)
+	}
+}
+
 func (w *x11Window) ShowTextInput(show bool) {}
 
 // Close the window.
@@ -612,6 +630,9 @@ func newX11Window(gioWin Callbacks, opts *Options) error {
 	w.atoms.clipboardContent = w.atom("CLIPBOARD_CONTENT", false)
 	w.atoms.atom = w.atom("ATOM", false)
 	w.atoms.targets = w.atom("TARGETS", false)
+	w.atoms.wmName = w.atom("_NET_WM_NAME", false)
+	w.atoms.wmState = w.atom("_NET_WM_STATE", false)
+	w.atoms.wmStateFullscreen = w.atom("_NET_WM_STATE_FULLSCREEN", false)
 
 	// set the name
 	ctitle := C.CString(opts.Title)
@@ -625,11 +646,13 @@ func newX11Window(gioWin Callbacks, opts *Options) error {
 			format:   8,
 			nitems:   C.ulong(len(opts.Title)),
 		},
-		w.atom("_NET_WM_NAME", false))
+		w.atoms.wmName)
 
 	// extensions
 	C.XSetWMProtocols(dpy, win, &w.atoms.evDelWindow, 1)
 
+	w.SetWindowMode(opts.WindowMode)
+
 	// make the window visible on the screen
 	C.XMapWindow(dpy, win)