feat(picker.preview): add option to opt out of real-buffer preview

[fmorroni]

Description

Picker previews can be of two types: scratch buffers or real buffers. Scratch buffers are used when the file isn't already open, while real buffers are used when the file is already loaded.

Using a real buffer means the preview inherits the buffer's existing state and keymaps, which can interfere with picker-specific keymaps (see #1417). It also updates the buffer's last-used timestamp, which can affect picker sorting (see #773).

For my use cases, having the real-buffer preview has no benefits. Other users also seem to feel the same way (#773 (comment)).

This PR adds a use_real_buffer option to allow users to opt out of real-buffer previews and always use scratch buffers instead. You would opt out by setting picker.previewers.file.use_real_buffer = false.

Related Issue(s)

#1417
#773

[dpetka2001]

You might also want to make the following addition

diff --git a/lua/snacks/picker/source/buffers.lua b/lua/snacks/picker/source/buffers.lua
index f7a7ee7a..47e9f349 100644
--- a/lua/snacks/picker/source/buffers.lua
+++ b/lua/snacks/picker/source/buffers.lua
@@ -47,6 +47,9 @@ function M.buffers(opts, ctx)
   end
   if opts.sort_lastused then
     table.sort(items, function(a, b)
+      if a.info.lastused == b.info.lastused then
+        return a.buf < b.buf
+      end
       return a.info.lastused > b.info.lastused
     end)
   end

because otherwise the order will still be non-deterministic for same lastused timestamps. For example I opened Neovim with multiple buffers, but only the first one was loaded. The rest had lastused = 0 and table.sort does not return the same order if you delete a buffer for example. And after you delete a buffer the picker refreshes and table.sort sorts items again but possibly not with same order always if all have same lastused value. We need a tiebreaker.

[fmorroni]

Hmm yeah that makes sense. Although I think that change should be added regardless of whether this PR is accepted or not, so maybe it should go in its own PR?

That being said, I have no issues with adding it here if that seems best.

[dpetka2001]

I'm just a simple user myself like you. I just made the suggestion because you mentioned in your OP about how lastused affects picker sort order. Without this change the sort order will still be affected.

Whether you want to make a different PR or add it here is up to you. Not making any decisions myself.