types.CaptureOptions.alternateScreen

Not what that program is displaying — an ordinary capture already reads that, because the program's display is the pane's visible grid. tmux saves the *normal* screen when a program switches to the alternate one (screen_alternate_on copies the grid into saved_grid, which is what -a reads), so this answers with what an editor or a pager is covering up, not with the editor or the pager.

It exists only while such a program is running: tmux frees the saved grid when the program leaves the alternate screen, and fails with "no alternate screen" whenever there is nothing on it.