diff --git a/crates/tty7-cli/src/commands.rs b/crates/tty7-cli/src/commands.rs
index 10b88fc3..ce9b8d04 100644
--- a/crates/tty7-cli/src/commands.rs
+++ b/crates/tty7-cli/src/commands.rs
@@ -772,7 +772,10 @@ fn adopt_pane(
 }
 
 /// The workspace a pane was spawned for, if it is still on this machine.
-fn owner_of(info: &tty7_core::daemon::protocol::PaneInfo, machine: &Machine) -> Option<WorkspaceId> {
+fn owner_of(
+    info: &tty7_core::daemon::protocol::PaneInfo,
+    machine: &Machine,
+) -> Option<WorkspaceId> {
     let owner = info.owner.as_deref()?;
     machine
         .workspaces
@@ -2201,7 +2204,9 @@ mod tests {
             .push_back(ReplyOk::TabTree(Box::new(Tab::leaf(37))));
 
         run_cli(
-            &["tty7", "tab", "new", "web", "--pane", "%37", "--cwd", "C:\\else"],
+            &[
+                "tty7", "tab", "new", "web", "--pane", "%37", "--cwd", "C:\\else",
+            ],
             &Context::default(),
             &mut backend,
         );
diff --git a/crates/tty7-core/src/core/machine.rs b/crates/tty7-core/src/core/machine.rs
index 5479709c..2a0fdb29 100644
--- a/crates/tty7-core/src/core/machine.rs
+++ b/crates/tty7-core/src/core/machine.rs
@@ -1304,12 +1304,9 @@ fn backup_path(path: &Path, generation: usize) -> PathBuf {
 /// Returns `Ok(())` when there was nothing to do: no tree yet, or the newest
 /// generation is younger than `spacing`.
 fn keep_a_generation(path: &Path, spacing: Duration) -> io::Result<()> {
-    let bytes = match std::fs::read(path) {
-        Ok(bytes) => bytes,
-        // Nothing has been written yet, so there is no previous generation.
-        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(()),
-        Err(e) => return Err(e),
-    };
+    // Asked before the tree is read: this runs on every persist and answers
+    // "nothing to do" on almost all of them, so reading the whole document
+    // first would be a full read per write for one copy every five minutes.
     let newest = backup_path(path, 0);
     let too_soon = std::fs::metadata(&newest)
         .and_then(|meta| meta.modified())
@@ -1318,6 +1315,12 @@ fn keep_a_generation(path: &Path, spacing: Duration) -> io::Result<()> {
     if too_soon {
         return Ok(());
     }
+    let bytes = match std::fs::read(path) {
+        Ok(bytes) => bytes,
+        // Nothing has been written yet, so there is no previous generation.
+        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(()),
+        Err(e) => return Err(e),
+    };
     // Oldest first, so nothing is overwritten before it has been moved down.
     // A generation that is not there yet simply has nothing to move.
     for generation in (1..BACKUP_GENERATIONS).rev() {
@@ -2701,8 +2704,12 @@ mod tests {
         let store = MachineStore::open(&path);
 
         let ws = store.workspace_create(None, None, None).unwrap();
-        store.tab_create(ws.id, None, seed(1, "/work"), None, None).unwrap();
-        store.tab_create(ws.id, None, seed(2, "/work"), None, None).unwrap();
+        store
+            .tab_create(ws.id, None, seed(1, "/work"), None, None)
+            .unwrap();
+        store
+            .tab_create(ws.id, None, seed(2, "/work"), None, None)
+            .unwrap();
 
         let kept = backup_path(&path, 0);
         assert!(
diff --git a/src/ui/tree_sync.rs b/src/ui/tree_sync.rs
index 524e1b60..be7f0ef3 100644
--- a/src/ui/tree_sync.rs
+++ b/src/ui/tree_sync.rs
@@ -2909,7 +2909,8 @@ mod tests {
             assert_eq!(parked.name, "deploy");
             assert_eq!(
                 parked.workspace, ws,
-                "and it is parked against the workspace it was typed for, so a window                  that walks into a different one cannot spend it (#716)"
+                "and it is parked against the workspace it was typed for, so a window \
+                 that walks into a different one cannot spend it (#716)"
             );
         });
     }