messagegui: fix keep idxSpan.start=0

Author: thyttan

apps/messagegui/app.js

@@ -337,7 +337,9 @@ function showMessagesScroller(msg, persist, alreadyProcessed) {
     lines : allLines,
     titleLines : titleLines,
     idxSpan: {
-      start : Math.min(idxSpan.start, alreadyProcessed.idxSpan.start||MESSAGES.length),
+      start : Math.min(idxSpan.start,
+        (alreadyProcessed.idxSpan.start===undefined) ?
+          MESSAGES.length : alreadyProcessed.idxSpan.start),
       stop : Math.max(idxSpan.stop, alreadyProcessed.idxSpan.stop||0)}
   };