Make HdyHeaderGroup support HdyHeaderBar
We unfortunately had to fork
HdyHeaderBar to add a few features we need, and extending
GtkHeaderBar was not an option. This makes that
HdyHeaderGroup doesn't support our own header bar type.
HdyHeaderBar without breaking its API (not just the C one but the ones generated via introspection) is complicated… especially if we want to keep it clean. That being said, while
HdyHeaderGroup has been designed to solve issues with multi-column apps,
HdyHeaderBar has been designed to solve issues with mono-column apps mainly, so having
HdyHeaderGroup not supportting
HdyHeaderBar right now may not be that big of an issue.
With all that in mind, I suggest to not touch
HdyHeaderGroup for 0.0.10, and instead to take 1.0 as an opportunity to change its API to use
GtkWidget rather than
GtkHeaderBar and to internally check that the widget is one of the two supported header bar formats.