added comments.

server/CXWindowsPrimaryScreen.cpp

@@ -597,7 +597,9 @@ CXWindowsPrimaryScreen::onLostClipboard(ClipboardID id)
 void
 CXWindowsPrimaryScreen::selectEvents(Display* display, Window w) const
 {
-	// ignore errors while we adjust event masks
+	// ignore errors while we adjust event masks.  windows could be
+	// destroyed at any time after the XQueryTree() in doSelectEvents()
+	// so we must ignore BadWindow errors.
 	CXWindowsUtil::CErrorLock lock;
 
 	// adjust event masks
@@ -611,13 +613,28 @@ CXWindowsPrimaryScreen::doSelectEvents(Display* display, Window w) const
 	// that we select PointerMotionMask on every window.  we also select
 	// SubstructureNotifyMask in order to get CreateNotify events so we
 	// select events on new windows too.
+	//
+	// note that this can break certain clients due a design flaw of X.
+	// X will deliver a PointerMotion event to the deepest window in the
+	// hierarchy that contains the pointer and has PointerMotionMask
+	// selected by *any* client.  if another client doesn't select
+	// motion events in a subwindow so the parent window will get them
+	// then by selecting for motion events on the subwindow we break
+	// that client because the parent will no longer get the events.
+
+	// FIXME -- should provide some workaround for event selection
+	// design flaw.  perhaps only select for motion events on windows
+	// that already do or are top-level windows or don't propagate
+	// pointer events.  or maybe an option to simply poll the mouse.
 
 	// we don't want to adjust our grab window
 	if (w == m_window) {
 		return;
 	}
 
-	// select events of interest
+	// select events of interest.  do this before querying the tree so
+	// we'll get notifications of children created after the XQueryTree()
+	// so we won't miss them.
 	XSelectInput(display, w, PointerMotionMask | SubstructureNotifyMask);
 
 	// recurse on child windows