diff --git a/common/rfb/VNCSConnectionST.cxx b/common/rfb/VNCSConnectionST.cxx
index f45ee7e..c63e6b6 100644
--- a/common/rfb/VNCSConnectionST.cxx
+++ b/common/rfb/VNCSConnectionST.cxx
@@ -1056,8 +1056,11 @@
 
     writeRTTPing();
 
+    // The request might be for just part of the screen, so we cannot
+    // just clear the entire update tracker.
+    updates.subtract(requested);
+
     requested.clear();
-    updates.clear();
   }
 
 out: