feat: use scratch buffer for file previews (#467)

* Initial implementation of scratch based preview

* Fix call to buf is valid in loop

* Fixing call to be made only from the main event loop

* Improve handling of large files from @pkazmier

* Better error handling and simplifying the code

* Default to old behavior

* Add documentation

* Fix readfile

* Fix the configuration

* refactor: single config enum and load real buffer on BufEnter

* doc: regenerate documentation

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

README.md

@@ -272,8 +272,8 @@ require("oil").setup({
   preview_win = {
     -- Whether the preview window is automatically updated when the cursor is moved
     update_on_cursor_moved = true,
-    -- Maximum file size in megabytes to preview
+    -- How to open the preview window "load"|"scratch"|"fast_scratch"
-    max_file_size_mb = 100,
+    preview_method = "fast_scratch",
     -- Window-local options to use for preview window buffers
     win_options = {},
   },

doc/oil.txt

@@ -157,8 +157,8 @@ CONFIG                                                                *oil-confi
       preview_win = {
         -- Whether the preview window is automatically updated when the cursor is moved
         update_on_cursor_moved = true,
-        -- Maximum file size in megabytes to preview
+        -- How to open the preview window "load"|"scratch"|"fast_scratch"
-        max_file_size_mb = 100,
+        preview_method = "fast_scratch",
         -- Window-local options to use for preview window buffers
         win_options = {},
       },

lua/oil/config.lua

@@ -142,8 +142,8 @@ local default_config = {
   preview_win = {
     -- Whether the preview window is automatically updated when the cursor is moved
     update_on_cursor_moved = true,
-    -- Maximum file size in megabytes to preview
+    -- How to open the preview window "load"|"scratch"|"fast_scratch"
-    max_file_size_mb = 100,
+    preview_method = "fast_scratch",
     -- Window-local options to use for preview window buffers
     win_options = {},
   },
@@ -326,16 +326,21 @@ local M = {}
 ---@field border? string|string[] Window border
 ---@field win_options? table<string, any>
 
+---@alias oil.PreviewMethod
+---| '"load"' # Load the previewed file into a buffer
+---| '"scratch"' # Put the text into a scratch buffer to avoid LSP attaching
+---| '"fast_scratch"' # Put only the visible text into a scratch buffer
+
 ---@class (exact) oil.PreviewWindowConfig
 ---@field update_on_cursor_moved boolean
----@field max_file_size_mb number
+---@field preview_method oil.PreviewMethod
 ---@field win_options table<string, any>
 
 ---@class (exact) oil.ConfirmationWindowConfig : oil.WindowConfig
 
 ---@class (exact) oil.SetupPreviewWindowConfig
 ---@field update_on_cursor_moved? boolean Whether the preview window is automatically updated when the cursor is moved
----@field max_file_size_mb? number Maximum file size in megabytes to preview
+---@field preview_method? oil.PreviewMethod How to open the preview window
 ---@field win_options? table<string, any> Window-local options to use for preview window buffers
 
 ---@class (exact) oil.SetupConfirmationWindowConfig : oil.SetupWindowConfig

lua/oil/init.lua

@@ -452,13 +452,6 @@ M.open_preview = function(opts, callback)
   if not entry then
     return finish("Could not find entry under cursor")
   end
-  if entry.meta ~= nil and entry.meta.stat ~= nil then
-    if entry.meta.stat.size >= config.preview_win.max_file_size_mb * 1e6 then
-      return finish(
-        "File over " .. config.preview_win.max_file_size_mb .. "MB is too large to preview"
-      )
-    end
-  end
   local entry_title = entry.name
   if entry.type == "directory" then
     entry_title = entry_title .. "/"
@@ -529,15 +522,20 @@ M.open_preview = function(opts, callback)
       end
     end
 
-    local filebufnr = vim.fn.bufadd(normalized_url)
     local entry_is_file = not vim.endswith(normalized_url, "/")
+    local filebufnr
+    if entry_is_file and config.preview_win.preview_method ~= "load" then
+      filebufnr =
+        util.read_file_to_scratch_buffer(normalized_url, config.preview_win.preview_method)
+    end
 
-    -- If we're previewing a file that hasn't been opened yet, make sure it gets deleted after
+    if not filebufnr then
-    -- we close the window
+      filebufnr = vim.fn.bufadd(normalized_url)
       if entry_is_file and vim.fn.bufloaded(filebufnr) == 0 then
         vim.bo[filebufnr].bufhidden = "wipe"
         vim.b[filebufnr].oil_preview_buffer = true
       end
+    end
 
     ---@diagnostic disable-next-line: param-type-mismatch
     local ok, err = pcall(vim.cmd, {

lua/oil/util.lua

@@ -897,4 +897,59 @@ M.get_icon_provider = function()
   end
 end
 
+---Read a buffer into a scratch buffer and apply syntactic highlighting when possible
+---@param path string The path to the file to read
+---@param preview_method oil.PreviewMethod
+---@return nil|integer
+M.read_file_to_scratch_buffer = function(path, preview_method)
+  local bufnr = vim.api.nvim_create_buf(false, true)
+  if bufnr == 0 then
+    return
+  end
+
+  vim.bo[bufnr].bufhidden = "wipe"
+  vim.bo[bufnr].buftype = "nofile"
+
+  local max_lines = preview_method == "fast_scratch" and vim.o.lines or nil
+  local has_lines, read_res = pcall(vim.fn.readfile, path, "", max_lines)
+  local lines = has_lines and vim.split(table.concat(read_res, "\n"), "\n") or {}
+
+  local ok = pcall(vim.api.nvim_buf_set_lines, bufnr, 0, -1, false, lines)
+  if not ok then
+    return
+  end
+  local ft = vim.filetype.match({ filename = path, buf = bufnr })
+  if ft and ft ~= "" then
+    local lang = vim.treesitter.language.get_lang(ft)
+    if not pcall(vim.treesitter.start, bufnr, lang) then
+      vim.bo[bufnr].syntax = ft
+    else
+    end
+  end
+
+  -- Replace the scratch buffer with a real buffer if we enter it
+  vim.api.nvim_create_autocmd("BufEnter", {
+    desc = "oil.nvim replace scratch buffer with real buffer",
+    buffer = bufnr,
+    callback = function()
+      local winid = vim.api.nvim_get_current_win()
+      -- Have to schedule this so all the FileType, etc autocmds will fire
+      vim.schedule(function()
+        if vim.api.nvim_get_current_win() == winid then
+          vim.cmd.edit({ args = { path } })
+
+          -- If we're still in a preview window, make sure this buffer still gets treated as a
+          -- preview
+          if vim.wo.previewwindow then
+            vim.bo.bufhidden = "wipe"
+            vim.b.oil_preview_buffer = true
+          end
+        end
+      end)
+    end,
+  })
+
+  return bufnr
+end
+
 return M