Fix scroll in tmux with max scrollback

I believe this is related to when scrollBottom is not the last row in the viewport

Fixes #434

src/xterm.js

@@ -1234,6 +1234,15 @@ Terminal.prototype.showCursor = function() {
 Terminal.prototype.scroll = function() {
   var row;
 
+  // Make room for the new row in lines
+  if (this.lines.length === this.lines.maxLength) {
+    this.lines.trimStart(1);
+    this.ybase--;
+    if (this.ydisp !== 0) {
+      this.ydisp--;
+    }
+  }
+
   this.ybase++;
 
   // TODO: Why is this done twice?
@@ -1248,13 +1257,6 @@ Terminal.prototype.scroll = function() {
   row -= this.rows - 1 - this.scrollBottom;
 
   if (row === this.lines.length) {
-    // Compensate ybase and ydisp if lines has hit the maximum buffer size
-    if (this.lines.length === this.lines.maxLength) {
-      this.ybase--;
-      if (this.ydisp !== 0) {
-        this.ydisp--;
-      }
-    }
     // Optimization: pushing is faster than splicing when they amount to the same behavior
     this.lines.push(this.blankLine());
   } else {