bug#81583: 32.0.50; Optional preview of the replacement text while typing it

> Personally, I'd prefer to keep the same "topic" in a single bug thread,
> if you're okay with committing 'bug#81583' in several commits.

This would be nice.

> Would you mind listing the features you had in mind?

1. We could add a special value 'both' of the new option
'query-replace-show-preview' to customize the display of
both strings side by side (match → replacement).

2. Another feature that could be added is to have a variable
that contains a string to display for zero-length matches.
Like a vertical bar for empty rectangular region after 'C-x SPC'.