bug#48841: fido-mode is slower than ido-mode with similar settings

[Dmitry Gutov <dgutov@yandex.ru>]

On 06.06.2021 02:20, João Távora wrote:
> Now, what I called here the "suffix-calculation logic" is what I also
> called the "[mplete] dance" back in the emacs-devel thread.  Truth is,
> it's always annoyed me in icomplete partially because I don't understand
> what it does exactly and how it is supposed to help me.  I suppose
> Stefan knows best here.  Regardless of its use, it seems to require
> another try-completion call in all the filtered candidates (which might
> be very big) so that's probably where the extra lag comes from.

Shouldn't this logic (whether it's used) be governed by the variable 
icomplete-hide-common-prefix?

Which icomplete--fido-mode-setup sets to nil (appropriately, given than 
ido-mode does not have this behavior). And looking at its behavior, it 
only does the "[mplete] dance" when there is only one match remaining.

Whether to make icomplete-hide-common-prefix affect even the "only one 
match" case is a matter of taste: I could personally find an argument 
for either choice. But we're seeing performance degradation when there 
are many matches, not one.