Questionable need for GtkHeaderBar::has-subtitle property
I noticed at least several Gnome apps, which reserve subtitle space without reason. I noticed, that e.g. in glade UI editor - GtkHeaderBar item has this option enabled by default. I just can't imagine, why should it ever be done - wasting space for no reason.
Example of such apps: Devhelp, Glade (unless multiple projects are opened), Gnome Sound Recorder, Yelp.
My wild guess - they just forgot to turn off space wasting. And if feature removal mania is standard in gnome, than "has-subtitle" removal is unlikely to be even noticed. If one wants to force something to be huge, "min-height" is enough (not only in pix, but for example in mm, cm, etc).
If subtitle present, there must be user option (in settings.ini or dconf) to choose, how to display subtitle: it could be on its own line (okay, let it be default) or just attached to main title, though still with specific style. There could be option about policy, defining when to move it to second line. Example variants: never, always, on-use, no-hspace. Simple 'auto' is not here, since there are already two auto-like policies.