Bug 281551 - [gtk2] Drop down lists do not open immediatly with certain CSS configuration

patch by robin.lu@sun.com r=bryner sr=roc

widget/src/gtk2/nsWindow.cpp

@@ -501,10 +501,8 @@ NS_IMETHODIMP
 nsWindow::IsVisible(PRBool & aState)
 {
     aState = mIsVisible;
-    if (mIsTopLevel && mShell && !GTK_WIDGET_MAPPED(mShell)) {
-        /* we do not change mIsVisible to PR_FALSE here so we don't bother
-           to change it back to PR_TRUE when the mShell is mapped again. */
-        aState = PR_FALSE;
+    if (mIsTopLevel && mShell) {
+        aState = GTK_WIDGET_VISIBLE(mShell);
     }
     return NS_OK;
 }