Notable to this new Mutter release is that it adds a --nested switch when running Mutter. Using this argument will allow running a nested Wayland session.
The nested switch is for restoring the Mutter Wayland session support, per this GNOME bug report that's been ongoing about the nested Wayland session for months.
GNOME Mutter 3.19.91 also fixes some stack scene graph stacking synchronization issues, primary selection protocol support for Wayland, and various bug fixes. The Wayland primary selection protocol work was covered a few days ago in GNOME's Mutter Gets Primary Selection Protocol For Wayland.
More details on the Mutter 3.19.91 release can be found via the Git change-log. GNOME 3.20 is shaping up to be an exciting release and further polishes the desktop environment's native Wayland support.