Update CenterIn() and CenterOnScreen() methods in BWindow

* These methods now return the new point after centering.
* But more importantly CenterIn() does some new adjustments to keep the window
  position inside the screen edge. If you pass the screen frame into CenterIn()
  it skips these adjustments.

headers/os/interface/Window.h

@@ -166,8 +166,8 @@ public:
 			void				ResizeBy(float dx, float dy);
 			void				ResizeTo(float width, float height);
 
-			void 				CenterIn(const BRect& rect);
-			void 				CenterOnScreen();
+			BPoint				CenterIn(const BRect& rect);
+			BPoint				CenterOnScreen();
 
 	virtual	void				Show();
 	virtual	void				Hide();

src/kits/interface/Window.cpp

@@ -2584,23 +2584,62 @@ BWindow::ResizeTo(float width, float height)
 }
 
 
-void
+/*!
+	\brief Center the window in the passed in rectangular area.
+	\param rect The rectangle to center the window in.
+	\return The new window position.
+*/
+BPoint
 BWindow::CenterIn(const BRect& rect)
 {
 	// Set size limits now if needed
 	UpdateSizeLimits();
 
-	MoveTo(BLayoutUtils::AlignInFrame(rect, Size(),
+	BPoint point = BLayoutUtils::AlignInFrame(rect, Size(),
 		BAlignment(B_ALIGN_HORIZONTAL_CENTER,
-			B_ALIGN_VERTICAL_CENTER)).LeftTop());
+			B_ALIGN_VERTICAL_CENTER)).LeftTop();
+
+	BRect screenFrame = (BScreen(this)).Frame();
+	if (screenFrame == rect) {
+		// if rect is the screen frame we can skip the ajustments below
+		MoveTo(point);
+		return point;
+	}
+
+	float borderWidth;
+	float tabHeight;
+	_GetDecoratorSize(&borderWidth, &tabHeight);
+
+	// clip the x position within the horizontal edges of the screen
+	if (point.x < screenFrame.left + borderWidth)
+		point.x = screenFrame.left + borderWidth;
+	else if (point.x + Frame().Width() > screenFrame.right - borderWidth)
+		point.x = screenFrame.right - borderWidth - Frame().Width();
+
+	// If the window is covering the window tab lower it down
+	if (point.y < rect.LeftTop().y + borderWidth)
+		point.y = rect.LeftTop().y + borderWidth;
+
+	// clip the y position within the vertical edges of the screen
+	if (point.y < screenFrame.top + borderWidth)
+		point.y = screenFrame.top + borderWidth;
+	else if (point.y + Frame().Height() > screenFrame.bottom - borderWidth)
+		point.y = screenFrame.bottom - borderWidth - Frame().Height();
+
+	MoveTo(point);
+	return point;
 }
 
 
-void
+/*!
+	\brief Centers the window on the screen the window is currently on.
+	\return The new window position.
+*/
+BPoint
 BWindow::CenterOnScreen()
 {
 	BScreen screen(this);
-	CenterIn(screen.Frame());
+	return CenterIn(screen.Frame());
 }