check scrollable elements for "clean" sides for CSR rendering.

lib/widget.js

@@ -651,6 +651,53 @@ Screen.prototype.deleteTop = function(top, bottom) {
   return this.deleteLine(1, top, top, bottom);
 };
 
+// Not exactly sure how worthwile this is.
+// This will cause a performance/cpu-usage hit,
+// but will it be less or greater than the
+// performance hit of slow-rendering scrollable
+// boxes with clean sides?
+Screen.prototype.cleanSides = function(el) {
+  if (!el._lastPos) {
+    return false;
+  }
+
+  var pos = el._lastPos
+    , yi = pos.yi + (el.border ? 1 : 0) + el.padding
+    , yl = pos.yl - (el.border ? 1 : 0) - el.padding
+    , first = this.olines[yi][pos.xi - 1]
+    , ch
+    , x
+    , y;
+
+  if (pos._cleanSides != null) {
+    return pos._cleanSides;
+  }
+
+  if (pos.xi === 0 && pos.xl === this.width) {
+    return pos._cleanSides = true;
+  }
+
+  for (; y < yl; y++) {
+    for (x = pos.xi - 1; x >= 0; x--) {
+      ch = this.olines[y][x];
+      if (ch[0] !== first[0] || ch[1] !== first[1]) {
+        return pos._cleanSides = false;
+      }
+    }
+  }
+
+  for (; y < yl; y++) {
+    for (x = pos.xl; x < this.width; x++) {
+      ch = this.olines[y][x];
+      if (ch[0] !== first[0] || ch[1] !== first[1]) {
+        return pos._cleanSides = false;
+      }
+    }
+  }
+
+  return pos._cleanSides = true;
+};
+
 Screen.prototype.draw = function(start, end) {
   var x
     , y
@@ -1960,6 +2007,7 @@ outer:
 
 Box.prototype.insertLine = function(i, line) {
   //if (typeof line === 'string') line = [line];
+  // if (this.screen.cleanSides(this)) {
   if (this._lastPos && this._lastPos.xi === 0 && this._lastPos.xl === this.screen.width) {
     i = Math.max(i, 0);
     i = Math.min(i, this._lastPos.yl - this._lastPos.yi - (this.border ? 2 : 0) - this.padding * 2);
@@ -1980,6 +2028,7 @@ Box.prototype.insertLine = function(i, line) {
 Box.prototype.deleteLine = function(i) {
   //if (typeof line === 'string') line = [line];
   var reset = true;
+  // if (this.screen.cleanSides(this)) {
   if (this._lastPos && this._lastPos.xi === 0 && this._lastPos.xl === this.screen.width) {
     i = Math.max(i, 0);
     i = Math.min(i, this._lastPos.yl - this._lastPos.yi - (this.border ? 2 : 0) - this.padding * 2);
@@ -2156,6 +2205,7 @@ ScrollableBox.prototype.scroll = function(offset) {
 
   // Optimize scrolling with CSR + IL/DL.
   p = this._lastPos;
+  // if (this.childBase !== base && this.screen.cleanSides(this)) {
   if (this.childBase !== base && p && (p.xl - p.xi) === this.screen.width) {
     t = p.yi + (this.border ? 1 : 0) + this.padding;
     b = p.yl - (this.border ? 1 : 0) - this.padding - 1;