How do I diff two files that are already open in Vim splits?

Explanation

You often have two files open side by side and want to compare them without leaving Vim or launching vimdiff. The :diffthis command turns the current window into a diff participant. Run it in each of two split windows, and Vim instantly highlights the differences between them with full diff navigation.

How it works

• :diffthis — marks the current window as part of a diff. Run this in each window you want to compare
• Once two or more windows are marked, Vim activates diff mode: color-coded change highlighting, folded unchanged regions, and synchronized scrolling
• :diffoff — removes diff mode from the current window
• :diffoff! — removes diff mode from all windows

Example

With two files open in vertical splits:

" In the left window:
:diffthis

" Switch to the right window:
<C-w>l
:diffthis

Vim immediately shows additions, deletions, and modifications highlighted. Navigate between changes with ]c (next change) and [c (previous change).

Tips

• Use :diffget (do) and :diffput (dp) to merge individual changes between windows
• :diffupdate forces Vim to recalculate the diff after manual edits
• This also works for comparing a buffer against a scratch buffer — paste clipboard content into a new split, then diff both
• For three-way merges, you can run :diffthis in three windows simultaneously