How do I show documentation for the selected completion item in a popup window?

Explanation

Adding popup to completeopt makes Vim display extra information — such as function signatures or documentation — for the currently highlighted completion item in a floating popup window, rather than opening a split preview window.

How it works

When the completion menu is open and you move through candidates, Vim checks whether the completion source provides an info field for the item. If it does, that text is shown in a small popup to the side of the menu.

• popup — show info in a floating popup window (Vim 8.1.1882+ and Neovim 0.10+)
• preview — the older alternative; opens a split window at the top

To enable it alongside other common options:

set completeopt=menu,menuone,noselect,popup

Example

With set omnifunc=syntaxcomplete#Complete in a Python file, pressing <C-x><C-o> opens a completion menu. Selecting os.path.join shows its signature and documentation in a compact popup next to the menu — no split window needed.

Tips

• To also suppress the split window being opened for preview, use noselect,popup together
• The popup uses the CompleteInfo highlight group for styling
• If an item has no info, the popup does not appear — this is expected
• Requires Vim 8.1.1882+ or Neovim 0.10+; use preview as a fallback for older versions
• LSP clients (like the built-in Neovim LSP) populate the info field automatically, making this very useful in LSP workflows