Choose a viewer to always preview

Designating a viewer as the default panel to preview is especially useful when you have a Composition viewer that represents your final output and you always want to preview that viewer even when you’re changing settings in other panels.

The panel that’s set to always preview appears frontmost for the duration of the preview.

 Click the Always Preview This View button  in the lower-left corner of the panel.
Note: When multiple views are open, previews use the frontmost composition view for 2D compositions and the Active Camera view for 3D compositions. To turn off the Active Camera, deselect Previews Favor Active Camera in the Preview panel menu.