When switching tabs while an on-reflow initiated callback
might still be running can lead to bogus information
coming back from position-locations, as the editor loses
its admin.
So, we just give up recomputing the rectangles when the
admin is gone and, for now, expect that there will be
another on-reflow call when the admin comes back that'll
restart the process (not sure that this is guaranteed tho)
original commit: 52d0b7e352a493e127e0d7cd780a34938dddea08
d54e68a2e7