Index: src/hg/js/hgTracks.js
===================================================================
RCS file: /projects/compbio/cvsroot/kent/src/hg/js/hgTracks.js,v
retrieving revision 1.18
retrieving revision 1.19
diff -b -B -U 4 -r1.18 -r1.19
--- src/hg/js/hgTracks.js	21 Jan 2009 00:12:40 -0000	1.18
+++ src/hg/js/hgTracks.js	24 Feb 2009 22:47:21 -0000	1.19
@@ -91,9 +91,15 @@
         startDelta = Math.floor(mult * (x1 - insideX));
     }
     if(singleClick) {
 	var newStart = (winStart + 1) + (startDelta - Math.floor(newWinWidth / 2));
-	var newEnd = (winStart + 1) + (startDelta + Math.floor(newWinWidth / 2));
+        if(newStart < 1) {
+            newStart = 1;
+            newEnd = newWinWidth;
+        } else {
+            // hgTracks gracefully handles overflow past the end of the chrom, so don't worry about that.
+            newEnd = (winStart + 1) + (startDelta + Math.floor(newWinWidth / 2));
+        }
 	newPos = chromName + ":" + newStart + "-" + newEnd;
 	newSize = newEnd - newStart + 1;
     } else {
         var endDelta;

