Integration: keep another instance's shift tool switch choice (#5878)

Two instances migrate the same version 1 file (useShiftKeyForToolSwitch
false -> true), so both save bases hold false. When B turns the switch off
and saves version 2, A's next unrelated save wrote its migrated true over
that choice. write_prefs now drops the migration delta from the base when
the file on disk is already version 2 or later.

Test: another_instances_shift_tool_switch_choice_beats_this_ones_migration
(fails without the fix: true written over false).

Co-authored-by: Higgs32584 <55243596+Higgs32584@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/ui-egui/src/prefs_ui.rs

@@ -618,6 +618,7 @@ fn write_prefs(app: &mut PhotocraftApp, ours: &Value) -> Result<(), String> {
     let stored = app.services.load_prefs.as_mut().and_then(|load| load()).and_then(|t| serde_json::from_str::<Value>(&t).ok());
     let text = match (stored, &app.prefs_rt.saved_value) {
         (Some(mut theirs @ Value::Object(_)), Some(base)) => {
+            let base = &without_stale_migration(base, ours, &theirs);
             merge_changes(base, ours, &mut theirs);
             serde_json::to_string_pretty(&theirs)
         }
@@ -627,6 +628,21 @@ fn write_prefs(app: &mut PhotocraftApp, ours: &Value) -> Result<(), String> {
     save(&text)
 }
 
+/// The save base without the shift tool switch migration (#5647) once storage already holds a
+/// version 2 file: another instance wrote it, so its Use Shift Key for Tool Switch is a choice,
+/// not the old default this instance migrated, and must not be overwritten by the migration.
+fn without_stale_migration(base: &Value, ours: &Value, theirs: &Value) -> Value {
+    let mut base = base.clone();
+    let new_file = |v: &Value| v.get("version").and_then(Value::as_u64).is_some_and(|v| v >= 2);
+    if new_file(theirs) && !new_file(&base) {
+        let key = "useShiftKeyForToolSwitch";
+        if let (Some(o), Some(tools)) = (ours.pointer("/tools").and_then(|t| t.get(key)), base.get_mut("tools").and_then(Value::as_object_mut)) {
+            tools.insert(key.into(), o.clone());
+        }
+    }
+    base
+}
+
 /// Three-way merge of preference trees: whatever `ours` changed since `base` goes over `theirs`
 /// (what storage holds now) and the rest of `theirs` is kept. Objects merge key by key; any
 /// other value, arrays included, is replaced whole.
@@ -3276,6 +3292,26 @@ mod tests {
         assert_eq!((v["interface"]["theme"].as_str(), v["performance"]["historyStates"].as_u64()), (Some("studio"), Some(12)));
     }
 
+    #[test]
+    fn another_instances_shift_tool_switch_choice_beats_this_ones_migration() {
+        // Two instances migrate the same version 1 file. One turns the switch off and saves
+        // version 2; the other's unrelated save must not write its migrated "on" over that.
+        let saved = json!({"version": 1, "tools": {"useShiftKeyForToolSwitch": false}});
+        let store = Arc::new(Mutex::new(Some(saved.to_string())));
+        let ctx = egui::Context::default();
+        let (mut a, mut b) = (app_on(&store, Arc::default()), app_on(&store, Arc::default()));
+        tick(&mut a, &ctx);
+        tick(&mut b, &ctx);
+        b.run("prefs.set", json!({"values": {"tools.useShiftKeyForToolSwitch": false}})).unwrap();
+        tick(&mut b, &ctx);
+        assert_eq!(stored(&store)["tools"]["useShiftKeyForToolSwitch"], false);
+        a.run("prefs.set", json!({"values": {"performance.historyStates": 12}})).unwrap();
+        tick(&mut a, &ctx);
+        let v = stored(&store);
+        assert_eq!((v["tools"]["useShiftKeyForToolSwitch"].as_bool(), v["performance"]["historyStates"].as_u64()), (Some(false), Some(12)));
+        assert_eq!(v["version"], json!(prefs::PREFS_VERSION));
+    }
+
     #[test]
     fn a_newer_files_version_is_kept_on_save() {
         let saved = json!({"version": prefs::PREFS_VERSION + 1, "tools": {"useShiftKeyForToolSwitch": false}});