test(autosave): fix a flake: wait for the background write to finish, not just for its file

Since #2518 the write syncs the folder after the rename, so the autosave
file exists while the write is still running. Until it finishes the drawing
is busy and the next change isn't planned, so the test's tick() returned
None. Autosaver::writing() says whether a write is still running.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/autosave.rs

@@ -416,6 +416,13 @@ mod native {
             writes
         }
 
+        /// Whether a background write hasn't finished yet. Its file can exist before it finishes
+        /// (the folder is synced after the rename), and until [`Autosaver::reap`] takes it in, the
+        /// drawing is left out of planning.
+        pub fn writing(&self) -> bool {
+            self.running.iter().any(|(_, h)| !h.is_finished())
+        }
+
         /// Take in the writes that have finished ([`Autosaver::tick`] does this first). A drawing
         /// whose write failed is no longer counted as autosaved: the next plan restarts its clock,
         /// so it is tried again SAVETIME later even if it doesn't change.

crates/engine/tests/autosave.rs

@@ -185,6 +185,14 @@ fn autosave_files_are_written_in_the_background_and_removed_on_exit() {
         std::thread::sleep(Duration::from_millis(10));
     }
     assert!(std::fs::read(&path).unwrap().ends_with(b"EOF\n"));
+    // The file exists before the write finishes (the folder sync comes after the rename); until
+    // it has, the drawing is busy and the change drawn below would not be planned.
+    for _ in 0..500 {
+        if !a.writing() {
+            break;
+        }
+        std::thread::sleep(Duration::from_millis(10));
+    }
     // While the app runs, its lock keeps its files out of recovery. SAVETIME and SAVEFILEPATH
     // are system variables (process-wide, so set in this test only).
     assert_eq!(cadcraft_engine::sysvars::get(&s, "SAVETIME"), Some(json!(10)));