the icon shouldn't be 'highlighted' - that way you're seeing the tl as is.
from manual (which you should have):
Click the Split-Screen View button in the Video Preview window to turn
split-screen previews on or off.
Split-screen previews allow you to split the Video Preview window so you
can see your affected and unaffected video or your video and the
contents of the clipboard at the same time. Use split-screen previews to
fine-tune video effects or to match colors for color correction.
You can use split-screen previews to help match colors between clips.
When the split-screen preview mode is set to Clipboard, the Video
Preview and Video Scopes windows will display the frame you copied to
the clipboard and the current frame.
The Video Preview window temporarily enters split-screen preview mode
when you slip-trim a video event. This temporary split-screen mode
allows you to see the event's first and last frame as you trim. You can
toggle this preview mode by selecting or deselecting the Video Event
Edge Frames command on the View menu.