diff -r 5c4e2636c1a9 -r 8c0c75aa3ff4 mercurial/bdiff.c
--- a/mercurial/bdiff.c	Tue Nov 08 18:37:33 2016 +0100
+++ b/mercurial/bdiff.c	Tue Nov 08 18:37:33 2016 +0100
@@ -143,7 +143,7 @@
 			struct pos *pos,
 			 int a1, int a2, int b1, int b2, int *omi, int *omj)
 {
-	int mi = a1, mj = b1, mk = 0, i, j, k, half;
+	int mi = a1, mj = b1, mk = 0, i, j, k, half, bhalf;
 
 	/* window our search on large regions to better bound
 	   worst-case performance. by choosing a window at the end, we
@@ -152,6 +152,7 @@
 		a1 = a2 - 30000;
 
 	half = (a1 + a2 - 1) / 2;
+	bhalf = (b1 + b2 - 1) / 2;
 
 	for (i = a1; i < a2; i++) {
 		/* skip all lines in b after the current block */
@@ -187,8 +188,8 @@
 					/* same match but closer to half */
 					mi = i;
 					mj = j;
-				} else if (i == mi) {
-					/* same i but earlier j */
+				} else if (i == mi && mj > bhalf) {
+					/* same i but best earlier j */
 					mj = j;
 				}
 			}