Don't allow blur values over 26 on high sierra. Issue 6115

sources/PTYTab.m

@@ -2173,7 +2173,13 @@ static void SetAgainstGrainDim(BOOL isVertical, NSSize *dest, CGFloat value) {
         }
     }
     if (count > 0) {
-        return sum / count;
+        if (@available(macOS 10.13, *)) {
+            // Issue 6115. It turns red for values over 26. When I drop 10.12 support I can adjust
+            // the slider to not go above 26.
+            return MIN(26, sum / count);
+        } else {
+            return sum / count;
+        }
     } else {
         // This shouldn't actually happen, but better safe than divide by zero.
         return 2.0;