This two button flyout contains Zoom Horizontal Extents and Zoom Horizontal Extents Keys.
Zoom Horizontal Extents: Adjusts the size of the Track View Edit Window horizontally so that all of the active time segment is visible at once.
Zoom Horizontal Extents Keys: Scales your view horizontally to display the full range of all animation keys. Depending on your animation, this view could be larger or smaller than the Active Time Segment.
The active time segment (displayed in a lighter color) centers itself in the edit window.