Look at the screenshot. Here 'A' is an overlay highlighted with font-lock-keyword-face
in buffer *scratch*. What happens if I type C-x C-e (eval-last-sexp) staying  where the
cursor is?

1) Redisplay is invoked with "xaxa" as current_buffer, and buffer_shared is recalculated
    to 2 because "xaxa" is displayed in 2 windows. On leaving redisplay_internal, buffer_shared
    is 2.

2) modify_overlay is called for "*scratch*", and this function still sees buffer_shared as 2
    (unchanged since last redisplay invocation), although *scratch* is displayed in the only
    window.

3) ???

IIUC, as the minimum minorum, buffer_shared should be reset to 0 after leaving redisplay_internal.
Or it should be a per-buffer field.

Dmitry