Unity Editor: Make a sub window wider than the parent window width.

In Unity Editor, I am trying to make a sub item(like a timeline) wider than its contained space, thus I can place a vertical scroll bar and let the end user scroll. For example:

Note the horizontal scroll bar. I was able to get it to appear, but I realized the portion of the window I'm attaching it to is only ever as wide as the visible area.

This is how I'm defining the area

 Rect rectFrameHeader = EditorGUILayout.BeginHorizontal(EditorStyles.toolbar, GUILayout.ExpandWidth(true));

I noticed rectFrameHeader.width is only the width of the available area(i.e. when I make the window smaller, its smaller).
When I tried to manually define the width, I get width of 0-12 regardless of what I set it to.

My question is how would I imitate the pictured behavior.