Made containers transparent on Windows. This is the first of a four-part set (though IDK if it will be four commits) to make both containers and areas both transparent and flicker-free. Also added a Space() tab to the test program to test the transparency.

redo/container_windows.c

@@ -15,6 +15,10 @@ static LRESULT CALLBACK containerWndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LP
 {
 	void *data;
 	RECT r;
+	HDC dc;
+	PAINTSTRUCT ps;
+	HWND parent;
+	POINT client;
 
 	data = (void *) GetWindowLongPtrW(hwnd, GWLP_USERDATA);
 	if (data == NULL) {
@@ -33,6 +37,29 @@ static LRESULT CALLBACK containerWndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LP
 		return forwardCommand(hwnd, uMsg, wParam, lParam);
 	case WM_NOTIFY:
 		return forwardNotify(hwnd, uMsg, wParam, lParam);
+	case WM_PAINT:
+		/* paint the parent's background in a flicker-free way */
+		dc = BeginPaint(hwnd, &ps);
+		if (dc == NULL)
+			abort();//TODO
+		parent = GetParent(hwnd);
+		if (parent == NULL)
+			abort();//TODO
+		if (GetWindowRect(hwnd, &r) == 0)
+			abort();//TODO
+		/* GetWindowRect() returns in screen coordinates; we want parent client */
+		client.x = r.left;
+		client.y = r.top;
+		if (ScreenToClient(parent, &client) == 0)
+			abort();//TODO
+		if (SetWindowOrgEx(dc, client.x, client.y, NULL) == 0)
+			abort();//TODO
+		SendMessageW(parent, WM_PRINTCLIENT, (WPARAM) dc, PRF_CLIENT);
+		EndPaint(hwnd, &ps);
+		return 0;
+	case WM_ERASEBKGND:
+		/* we paint our own background above */
+		return 1;
 	case WM_SIZE:
 		if (GetClientRect(hwnd, &r) == 0)
 			xpanic("error getting client rect for Window in WM_SIZE", GetLastError());
@@ -54,8 +81,7 @@ DWORD makeContainerWindowClass(char **errmsg)
 	wc.hInstance = hInstance;
 	wc.hIcon = hDefaultIcon;
 	wc.hCursor = hArrowCursor;
-	/* TODO won't this override our visual styles? */
-	wc.hbrBackground = (HBRUSH) (COLOR_BTNFACE + 1);
+	wc.hbrBackground = NULL;		/* we paint our own background */
 	wc.lpszClassName = containerclass;
 	if (RegisterClassW(&wc) == 0) {
 		*errmsg = "error registering container window class";
@@ -69,7 +95,7 @@ HWND newContainer(void *data)
 	HWND hwnd;
 
 	hwnd = CreateWindowExW(
-		0,
+		WS_EX_TRANSPARENT,
 		containerclass, L"",
 		WS_CHILD | WS_VISIBLE,
 		CW_USEDEFAULT, CW_USEDEFAULT,

redo/tab_windows.go

@@ -94,11 +94,9 @@ func (t *tab) commitResize(c *allocation, d *sizing) {
 	r.top = C.LONG(0)
 	r.right = C.LONG(c.width)
 	r.bottom = C.LONG(c.height)
-println(r.left, r.top, r.right, r.bottom)
 	C.tabGetContentRect(t._hwnd, &r)
 	// and resize tabs
 	// don't resize just the current tab; resize all tabs!
-println(r.left, r.top, r.right, r.bottom)
 	for _, c := range t.tabs {
 		// because each widget is actually a child of the Window, the origin is the one we calculated above
 		c.move(&r)

redo/zz_test.go

@@ -64,6 +64,7 @@ func (tw *testwin) make(done chan struct{}) {
 		done <- struct{}{}
 		return true
 	})
+	tw.t.Append("Space", Space())
 	tw.a = NewArea(200, 200, &areaHandler{})
 	tw.t.Append("Area", tw.a)
 	tw.spw = NewHorizontalStack(