Merge pull request #429 from Tyriar/428_scrollback_exception

Ensure ydisp does not drop below 0 in refresh

src/xterm.js

@@ -1251,7 +1251,9 @@ Terminal.prototype.scroll = function() {
     // Compensate ybase and ydisp if lines has hit the maximum buffer size
     if (this.lines.length === this.lines.maxLength) {
       this.ybase--;
-      this.ydisp--;
+      if (this.ydisp !== 0) {
+        this.ydisp--;
+      }
     }
     // Optimization: pushing is faster than splicing when they amount to the same behavior
     this.lines.push(this.blankLine());