Add some content to test panes.

regress/screen-redraw-popups.sh

@@ -42,7 +42,7 @@ compare() {
 	fi
 }
 
-C="sh -c 'exec sleep 100'"
+C="sh -c 'i=0; while [ \$i -lt 13 ]; do printf \"POP%02d abcdefghij\n\" \$i; i=\$((i + 1)); done; exec sleep 100'"
 
 # setup: fresh inner window attached inside a fresh outer pane, 40x14.
 setup() {