Function pinned_diff 

pub fn pinned_diff(
    vcs: &dyn Vcs,
    source: &ReviewSource,
    pr_head: Option<(&str, &str)>,
) -> Result<DiffModel, VcsError>

The freshly fetched diff for a commit, range, or PR review source, straight from the backend: the one place each variant’s vcs call is made, whether the caller reads it on the UI thread or off it. pr_head is the PR’s own (merge_base, head), resolved by the caller since a PR’s range lives in app state; None rejects an unresolved PR, so it is never silently read as unchanged. WorkingTree, Walkthrough, and Against carry no pinned diff of their own and read back empty.