app: add Maximize and Center methods support for macOS and X11

Commit 9835cd59 added support for the Window.Maximize and
Window.Center methods for Windows only.
This patch also adds support for macOS and X11.

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

app/os_x11.go

@@ -49,6 +49,11 @@ import (
 	"gioui.org/app/internal/xkb"
 )
 
+const (
+	_NET_WM_STATE_REMOVE = 0
+	_NET_WM_STATE_ADD    = 1
+)
+
 type x11Window struct {
 	w            *callbacks
 	x            *C.Display
@@ -81,6 +86,10 @@ type x11Window struct {
 		wmStateFullscreen C.Atom
 		// "_NET_ACTIVE_WINDOW"
 		wmActiveWindow C.Atom
+		// _NET_WM_STATE_MAXIMIZED_HORZ
+		wmStateMaximizedHorz C.Atom
+		// _NET_WM_STATE_MAXIMIZED_VERT
+		wmStateMaximizedVert C.Atom
 	}
 	stage  system.Stage
 	metric unit.Metric
@@ -241,37 +250,16 @@ func (w *x11Window) SetWindowMode(mode WindowMode) {
 	var action C.long
 	switch mode {
 	case Windowed:
-		action = 0 // _NET_WM_STATE_REMOVE
+		action = _NET_WM_STATE_REMOVE
 	case Fullscreen:
-		action = 1 // _NET_WM_STATE_ADD
+		action = _NET_WM_STATE_ADD
 	default:
 		return
 	}
 	w.config.Mode = mode
 	// "A Client wishing to change the state of a window MUST send
 	//  a _NET_WM_STATE client message to the root window."
-	var xev C.XEvent
-	ev := (*C.XClientMessageEvent)(unsafe.Pointer(&xev))
-	*ev = C.XClientMessageEvent{
-		_type:        C.ClientMessage,
-		display:      w.x,
-		window:       w.xw,
-		message_type: w.atoms.wmState,
-		format:       32,
-	}
-	arr := (*[5]C.long)(unsafe.Pointer(&ev.data))
-	arr[0] = action
-	arr[1] = C.long(w.atoms.wmStateFullscreen)
-	arr[2] = 0
-	arr[3] = 1 // application
-	arr[4] = 0
-	C.XSendEvent(
-		w.x,
-		C.XDefaultRootWindow(w.x), // MUST be the root window
-		C.False,
-		C.SubstructureNotifyMask|C.SubstructureRedirectMask,
-		&xev,
-	)
+	w.sendWMStateEvent(action, w.atoms.wmStateFullscreen, 0)
 }
 
 func (w *x11Window) ShowTextInput(show bool) {}
@@ -295,11 +283,54 @@ func (w *x11Window) Close() {
 	C.XSendEvent(w.x, w.xw, C.False, C.NoEventMask, &xev)
 }
 
-// Maximize the window. Not implemented for x11.
-func (w *x11Window) Maximize() {}
+// Maximize the window.
+func (w *x11Window) Maximize() {
+	w.sendWMStateEvent(_NET_WM_STATE_ADD, w.atoms.wmStateMaximizedHorz, w.atoms.wmStateMaximizedVert)
+}
 
-// Center the window. Not implemented for x11.
-func (w *x11Window) Center() {}
+// Center the window.
+func (w *x11Window) Center() {
+	screen := C.XDefaultScreen(w.x)
+	width := C.XDisplayWidth(w.x, screen)
+	height := C.XDisplayHeight(w.x, screen)
+
+	var attrs C.XWindowAttributes
+	C.XGetWindowAttributes(w.x, w.xw, &attrs)
+	width -= attrs.border_width
+	height -= attrs.border_width
+
+	sz := w.config.Size
+	x := (int(width) - sz.X) / 2
+	y := (int(height) - sz.Y) / 2
+
+	C.XMoveResizeWindow(w.x, w.xw, C.int(x), C.int(y), C.uint(sz.X), C.uint(sz.Y))
+}
+
+// action is one of _NET_WM_STATE_REMOVE, _NET_WM_STATE_ADD.
+func (w *x11Window) sendWMStateEvent(action C.long, atom1, atom2 C.ulong) {
+	var xev C.XEvent
+	ev := (*C.XClientMessageEvent)(unsafe.Pointer(&xev))
+	*ev = C.XClientMessageEvent{
+		_type:        C.ClientMessage,
+		display:      w.x,
+		window:       w.xw,
+		message_type: w.atoms.wmState,
+		format:       32,
+	}
+	data := (*[5]C.long)(unsafe.Pointer(&ev.data))
+	data[0] = C.long(action)
+	data[1] = C.long(atom1)
+	data[2] = C.long(atom2)
+	data[3] = 1 // application
+
+	C.XSendEvent(
+		w.x,
+		C.XDefaultRootWindow(w.x), // MUST be the root window
+		C.False,
+		C.SubstructureNotifyMask|C.SubstructureRedirectMask,
+		&xev,
+	)
+}
 
 var x11OneByte = make([]byte, 1)
 
@@ -735,6 +766,8 @@ func newX11Window(gioWin *callbacks, options []Option) error {
 	w.atoms.wmState = w.atom("_NET_WM_STATE", false)
 	w.atoms.wmStateFullscreen = w.atom("_NET_WM_STATE_FULLSCREEN", false)
 	w.atoms.wmActiveWindow = w.atom("_NET_ACTIVE_WINDOW", false)
+	w.atoms.wmStateMaximizedHorz = w.atom("_NET_WM_STATE_MAXIMIZED_HORZ", false)
+	w.atoms.wmStateMaximizedVert = w.atom("_NET_WM_STATE_MAXIMIZED_VERT", false)
 
 	// extensions
 	C.XSetWMProtocols(dpy, win, &w.atoms.evDelWindow, 1)