Fill inserted lines with background color

When scrolling occurs due to a LF or scroll code, the new line should be
filled with the currently set background color.  As with VTE, fill with
the _default_ color if the new line was caused by line-wrap.

src/Terminal.ts

@@ -1170,7 +1170,8 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
    * @param isWrapped Whether the new line is wrapped from the previous line.
    */
   public scroll(isWrapped?: boolean): void {
-    const newLine = BufferLine.blankLine(this.cols, DEFAULT_ATTR, isWrapped);
+    const blankAttr = isWrapped ? DEFAULT_ATTR : this.eraseAttr();
+    const newLine = BufferLine.blankLine(this.cols, blankAttr, isWrapped);
     const topRow = this.buffer.ybase + this.buffer.scrollTop;
     const bottomRow = this.buffer.ybase + this.buffer.scrollBottom;