Currently there are before and after comments in the page source to mark the begin and end of FTL template. Easier for development to display FTL file name and boundary on screen for a short time.
Only works for FTL files outside of theme folders, and when
1) print.verbose=true and widget.dev.namedBorder=true
OFBIZ-12018, widget.dev.namedBorder = NONE | LABEL | SOURCE