Re: [PATCH] Optional preview of the replacement text while typing it

> So I'm proposing an optional preview feature: while the replacement
> string is being read, the matches visible in the window are displayed as
> they would look after the replacement, using a new face
> `query-replace-preview'.

This feature is very much welcome.

> The patch is attached.

Thanks for the patch.

>   - The new option `query-replace-show-preview' defaults to nil.  Since
>     this is a visual change to the buffer, I think opting in is the
>     safer call, though it is debatable, and I am happy to flip it.

Agreed, nil by default is safer.

>   - The preview follows the same shape as the lazy highlight of the
>     search string (`minibuffer-lazy-highlight-setup' in isearch.el), and
>     only covers the matches visible in the selected window, so the work
>     per keystroke is bounded by a screenful.

Have you tried to generalize `minibuffer-lazy-highlight-setup'
to use for displaying replacements?  For example, by providing
a lambda to make replacements from the matches.  But probably
it's not possible to use the argument 'transform' since
every match might be transformed to different replacements.
So lazy-highlight would need to display different strings
instead of one search string like Isearch does.

>   - A replacement that is shorter or longer than the match shifts the
>     surrounding text while you type, as you can see happening to the
>     second column between screenshots 03 and 04.  The overlays use a
>     `display' property, so what you see is the width the buffer will
>     really have.

Understandable, this should be expected.

> I'd appreciate feedback on both the idea and the implementation. If
> there is interest in the feature, I'm happy to keep refining the patch.

I tried your patch, and it works nicely, thanks!

I have more suggestions for additional features, but
these could be added later.  When no one has more general
comments, please post the patch to the bug list for pushing.