From eee99f62e15f9449321b82bb890d47fc472d9743 Mon Sep 17 00:00:00 2001
From: Daniel Imms <daimms@microsoft.com>
Date: Thu, 9 Jun 2016 21:05:45 -0700
Subject: [PATCH] Improve scroll to work with blank lines after the cursor

This commit works fixes scrolling when there were blank lines after the cursor.
Here is what it does (blank rows are those added by running `clear`):

when increasing rows:
  if there are blank rows below the cursor:
    add a blank row to the bottom
  else if there is room in the buffer above the viewport
    scroll up
  else
    add a blank row to the bottom

when decreasing rows:
  if there are blank rows below the cursor:
    remove a blank row from the bottom
  else
    scroll down

Fixes #111
---
 src/xterm.js | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/src/xterm.js b/src/xterm.js
index b17437cc..f07dcc75 100644
--- a/src/xterm.js
+++ b/src/xterm.js
@@ -2607,7 +2607,8 @@
         , el
         , i
         , j
-        , ch;
+        , ch
+        , addToY;
 
       if (x === this.cols && y === this.rows) {
         return;
@@ -2639,11 +2640,23 @@
 
       // resize rows
       j = this.rows;
+      addToY = 0;
       if (j < y) {
         el = this.element;
         while (j++ < y) {
+          // y is rows, not this.y
           if (this.lines.length < y + this.ybase) {
-            this.lines.push(this.blankLine());
+            if (this.ybase > 0 && this.lines.length <= this.ybase + this.y + addToY + 1) {
+              // There is room above the buffer and there are no empty elements below the line,
+              // scroll up
+              this.ybase--;
+              this.ydisp--;
+              addToY++
+            } else {
+              // Add a blank line if there is no buffer left at the top to scroll to, or if there
+              // are blank lines after the cursor
+              this.lines.push(this.blankLine());
+            }
           }
           if (this.children.length < y) {
             this.insertRow();
@@ -2652,11 +2665,11 @@
       } else { // (j > y)
         while (j-- > y) {
           if (this.lines.length > y + this.ybase) {
-            if (this.y + this.ybase < j) {
-              // The line is after the cursor, remove it
+            if (this.lines.length > this.ybase + this.y + 1) {
+              // The line is a blank line below the cursor, remove it
               this.lines.pop();
             } else {
-              // The line is the cursor, push the viewport down
+              // The line is the cursor, scroll down
               this.ybase++;
               this.ydisp++;
             }
@@ -2676,6 +2689,9 @@
       if (this.y >= y) {
         this.y = y - 1;
       }
+      if (addToY) {
+        this.y += addToY;
+      }
 
       if (this.x >= x) {
         this.x = x - 1;