The newly-created session was stamped with the request's hidpi factor,
but `WebViewBuilder` defaults `hidpi_scale_factor` to 1.0 and we never
override it. `apply_viewport`'s diff check then saw
`session.dpr == request.dpr` and skipped `set_hidpi_scale`, leaving Servo
at hidpi=1.0 forever. On Retina that collapses CSS pixels onto device
pixels — the page lays out for a 2× viewport and renders at half size.
Same shape applies to page zoom.
Initialize the session with Servo's actual post-build defaults so the
viewport diff is the source of truth for whether `set_hidpi_scale` and
`set_page_zoom` need to run.
ZacharyZhang-NY
a067d62ad3