bug#34038: 26.1; set-window-start sometimes fails to set window start

[martin rudalics]

> A bit of debugging output shows that after (erase-buffer), window-start
> is already at 1, not the former point-max.

'window-start' returns the position of the window start marker.  When
you erase a buffer _all_ its markers go to point-min - point-max and
all window start or point markers included.

> After (insert b),
> window-start continues to be at 1.

Because the insertion type of a window start marker is nil.  You can
use 'insert-before-markers' to override that.

martin