Viewport properties have a bug.  With viewport selected (single click), it lists 5 properties in the properties screen.  These work correctly.

But when you double-click into the viewport, it goes all wonky.  When you manually zoom and pan around, the property numbers update reactively, and reflect the model space details.  But when you type into the dialog where you want to center model space etc, it updates the view location in paper space instead.  So it seems like the dialog either needs 10 properties, or to properly toggle back and forth between the viewport location in Paper space and the viewport view in Model space.

Workaround was to use Zoom Window with Grid Snap on to get it exactly right.  Object snap doesn't work with Zoom Window... so bonus bug.