Adjust BAlert to position the alert using the revamped BWindow::CenterIn() method.

BAlert::AlertPosition() has been superceded by the new BWindow method and should
be considered deprecated. Note that the alert position is not in the direct center of the
screen, it is offset vertically like before.

src/kits/interface/Alert.cpp

@@ -396,6 +396,7 @@ BAlert::QuitRequested()
 }
 
 
+// This method is deprecated, do not use
 BPoint
 BAlert::AlertPosition(float width, float height)
 {
@@ -572,7 +573,16 @@ BAlert::_InitObject(const char* text, const char* button0, const char* button1,
 
 	AddCommonFilter(new(std::nothrow) _BAlertFilter_(this));
 
-	MoveTo(AlertPosition(Frame().Width(), Frame().Height()));
+	// Position the alert so that it is centered vertically but offset a bit
+	// horizontally in the parent window's frame or, if unavailable, the
+	// screen frame.
+	BWindow* window =
+		dynamic_cast<BWindow*>(BLooper::LooperForThread(find_thread(NULL)));
+	const BRect frame = window != NULL ? window->Frame()
+		: (BScreen(this)).Frame();
+
+	CenterIn(frame.InsetByCopy(0, frame.Height() / 2)
+		.OffsetBySelf(0, -frame.Height() / 4));
 }