  We develop a framework based on the covariant phase space formalism that
identifies gravitational edge modes as dynamical reference frames. They enable
the identification of the associated spacetime region and the imposition of
boundary conditions in a gauge-invariant manner. While recent proposals
considered the finite region in isolation and sought the maximal symmetry
algebra compatible with that perspective, we regard it as a subregion embedded
in a global spacetime and study the symmetries consistent with such an
embedding. This clarifies that the frame, although appearing as "new" for the
subregion, is built out of the field content of the complement. Given a global
variational principle, this also permits us to invoke a systematic
post-selection procedure, previously used in gauge theory [arXiv:2109.06184],
to produce consistent dynamics for a subregion with timelike boundary.
Requiring the subregion presymplectic structure to be conserved by the dynamics
leads to an essentially unique prescription and unambiguous Hamiltonian
charges. Unlike other proposals, this has the advantage that all spacetime
diffeomorphisms acting on the subregion remain gauge and integrable, thus
generating a first-class constraint algebra. By contrast, diffeomorphisms
acting on the frame-dressed spacetime are physical, and those that are parallel
to the boundary are integrable. Further restricting to ones preserving the
boundary conditions yields an algebra of conserved charges. These record
changes in the relation between the region and its complement as measured by
frame reorientations. Finally, we explain how the boundary conditions and
presymplectic structure can be encoded into boundary actions. While our
formalism applies to any generally covariant theory, we illustrate it on
general relativity, and conclude with a detailed comparison of our findings to
earlier works. [abridged]
