fix: set alternate when using floating windows (#285)

lua/oil/init.lua

@@ -617,6 +617,25 @@ M.select = function(opts, callback)
     if callback then
       callback(err)
     end
+
+    -- Set alternate for buffer
+    local has_orig, orig_buffer = pcall(vim.api.nvim_win_get_var, 0, "oil_original_buffer")
+
+    if has_orig and vim.api.nvim_buf_is_valid(orig_buffer) then
+      if vim.api.nvim_get_current_buf() ~= orig_buffer then
+        -- If we are editing a new file after navigating around oil, set the alternate buffer
+        -- to be the last buffer we were in before opening oil
+        vim.fn.setreg("#", orig_buffer)
+      else
+        -- If we are editing the same buffer that we started oil from, set the alternate to be
+        -- what it was before we opened oil
+        local has_orig_alt, alt_buffer =
+          pcall(vim.api.nvim_win_get_var, 0, "oil_original_alternate")
+        if has_orig_alt and vim.api.nvim_buf_is_valid(alt_buffer) then
+          vim.fn.setreg("#", alt_buffer)
+        end
+      end
+    end
   end
   if not opts.split and (opts.horizontal or opts.vertical) then
     if opts.horizontal then

tests/altbuf_spec.lua

@@ -141,5 +141,17 @@ a.describe("Alternate buffer", function()
       oil.close()
       assert.equals("foo", vim.fn.expand("#"))
     end)
+
+    a.it("preserves alternate when traversing to a new file", function()
+      vim.cmd.edit({ args = { "foo" } })
+      oil.open_float()
+      test_util.wait_for_autocmd({ "User", pattern = "OilEnter" })
+      assert.equals("foo", vim.fn.expand("#"))
+      local last_item = vim.fn.line("$")
+      vim.api.nvim_win_set_cursor(0, { last_item, 5 })
+      oil.select()
+      test_util.wait_for_autocmd("BufEnter")
+      assert.equals("foo", vim.fn.expand("#"))
+    end)
   end)
 end)