Run preview command when preview window appears after resize (#3113)

# Start fzf in a small screen so that the preview window is hidden
  fzf --bind 'ctrl-p:toggle-preview' --preview 'stat {}' --preview-window='right,50%,<100(down,50%,hidden)'

  # Enlarge the screen until the preview window appears. It should not be empty.

src/terminal.go

@@ -2622,7 +2622,11 @@ func (t *Terminal) Loop() {
 						t.tui.Resume(t.fullscreen, t.sigstop)
 						t.redraw()
 					case reqFullRedraw:
+						wasHidden := t.pwindow == nil
 						t.redraw()
+						if wasHidden && t.pwindow != nil {
+							refreshPreview(t.previewOpts.command)
+						}
 					case reqClose:
 						exit(func() int {
 							if t.output() {