The private layout hook computes constraints and allocates viewport
space, showing only the required in-content bars. A corner button is
shown when both bars are needed. No notification wiring from that button
to the scroll offsets is installed here, so its presence does not
establish working two-axis drag scrolling.
Show reads content Left/Top/Width/Height, writes each bar's
Prop_First/Entries/Visible from offsets, content extents and inner
viewport dimensions, then returns Group Show's result. Getter/setter
results during synchronization are ignored. Range synchronization is
performed by this Show override, not by the private layout hook itself.
Notifications connect bar Prop_First changes to content offsets, and
content offset changes back to the bars. The callback uses NoNotify on
destination updates to prevent feedback. Bar-to-content writes also
disable Group child forwarding. Registration results are ignored.