diff of staged file with unstaged changes always shows staged changes rather than unstaged

[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.

Actual behavior: Only the staged changes are shown.