bug#60411: 29.0.60; minibuffer-next-completion skips first candidate when completions-header-format and completion-show-help are nil

[Gregory Heytings]

> That's very surprising to hear.  AFAIU, it just looks for some special 
> text property (in next-completion).  So it sounds like a very simple 
> breakage of logic, where "next" means "the first one" when you are 
> exactly at BOB.

The problem is that minibuffer-next-completion is supposed to move to the 
first completion when it is called for the first time, to the next 
completion when it is called for the Nth time, and to the first completion 
again when it is called for the Mth time (where M is the number of 
completion candidates).

If point is at BOB, and if there is nothing before the first completion, 
next-completion finds that there is a text property there, and therefore 
moves to the end of the current completion (the first one) and to the 
beginning of the next completion, with the two calls to 
next-single-property-change.

There is nothing in the current logic of the code with which it is 
possible to make a distinction between "this is the first call" and "this 
is not the first call".