diff of staged file with unstaged changes is wrong

[ctcjab]

1. Change a tracked file
2. Stage the file
3. Make further changes to the file
4. Hover over the file in the “changed” section and click the “diff” icon

Expected behavior: The diff between the staged version of the file and the unstaged version is shown, matching what is shown by the git diff command.

Actual behavior: The combined changes shown by gitt diff and git diff --staged are being shown: