This is a Textadept module for visualizing and merging the differences between two files. Install it in your ~/.textadept/modules/file_diff directory and load it from your ~/.textadept/init.lua:

_M.file_diff = require('file_diff')

By default the F8 key starts a diff session, prompting for two files to diff.

A sample workflow is this:

1. Start diff'ing two files via F8.
2. The caret is initially placed in the file on the left.
3. Go to the next change via Alt+Down.
4. Merge the change from the other buffer into the current one (right to left) via Alt+Left.
5. Go to the next change via Alt+Down.
6. Merge the change from the current buffer into the other one (left to right) via Alt+Right.
7. Repeat as necessary.

Textadept also provides these actions in a "Compare Files" submenu in the "Tools" menu.

Note: merging can be performed wherever the caret is placed when jumping between changes, even if one buffer has a change and the other does not (additions or deletions).