Integration: #927 Header & Footer dialog sends dateFormat only when picked; set_date doc comment back above set_date

Without it, Apply rewrote every slide's existing date field to datetime1.

crates/engine/src/cmd/design.rs

@@ -961,8 +961,6 @@ fn header_footer(s: &mut Session, p: &Value) -> Result<Value> {
     ok()
 }
 
-/// Point a slide's date placeholder at a fixed date (its own plain text) or, for an empty
-/// `fixed`, at an automatic date field (one it already has keeps its format).
 /// The date field type `format` names, `datetime1` when it is empty or unknown.
 fn date_kind(format: &str) -> &str {
     if deckcraft_text::datetime::DATE_KINDS.contains(&format) { format } else { "datetime1" }
@@ -974,6 +972,8 @@ fn date_now(format: &str) -> String {
     deckcraft_text::datetime::DateTime::now_local().and_then(|now| deckcraft_text::datetime::field_text(kind, None, now)).unwrap_or_else(today)
 }
 
+/// Point a slide's date placeholder at a fixed date (its own plain text) or, for an empty
+/// `fixed`, at an automatic date field (one it already has keeps its format).
 fn set_date(sh: &mut Shape, fixed: &str, format: &str) {
     let runs = || sh.text.iter().flat_map(|t| &t.paragraphs).flat_map(|p| &p.runs);
     let field_now = runs().find_map(|r| match &r.kind {

crates/ui-egui/src/dialogs.rs

@@ -1126,11 +1126,20 @@ fn body(app: &mut SlideApp, ui: &mut Ui, d: &mut Dialog) -> bool {
             if apply || apply_all {
                 let dt = d.get("dateText", "");
                 let ft = d.get("footerText", "");
-                run(
-                    app,
-                    "design.headerFooter",
-                    json!({"date": date, "dateText": dt, "dateFormat": d.get("dateFormat", &fmt_now), "slideNumber": num_on, "footer": foot, "footerText": ft, "hideOnTitle": hide, "all": apply_all}),
-                );
+                let mut params = serde_json::Map::new();
+                params.insert("date".into(), json!(date));
+                params.insert("dateText".into(), json!(dt));
+                // Only a format the user picked: sending the default would rewrite every slide's
+                // existing date field (an imported datetime4, say) to datetime1.
+                if d.fields.contains_key("dateFormat") {
+                    params.insert("dateFormat".into(), json!(d.get("dateFormat", &fmt_now)));
+                }
+                params.insert("slideNumber".into(), json!(num_on));
+                params.insert("footer".into(), json!(foot));
+                params.insert("footerText".into(), json!(ft));
+                params.insert("hideOnTitle".into(), json!(hide));
+                params.insert("all".into(), json!(apply_all));
+                run(app, "design.headerFooter", serde_json::Value::Object(params));
             }
             apply || apply_all || d.fields.contains_key("cancel")
         }
@@ -3410,6 +3419,55 @@ mod tests {
         h
     }
 
+    /// Header & Footer ▸ Apply without picking a format leaves a slide's existing date field in
+    /// its own format instead of rewriting it to the default one.
+    #[test]
+    fn header_footer_apply_keeps_an_existing_date_format() {
+        use std::sync::Arc;
+
+        use egui::vec2;
+        use egui_kittest::kittest::Queryable as _;
+        use serde_json::json;
+
+        use crate::SlideApp;
+        let mut session = deckcraft_engine::Session::with_new();
+        session.execute("design.headerFooter", &json!({"date": true, "dateText": "", "dateFormat": "datetime4", "all": true})).unwrap();
+        // A file whose document-level format is unset, as an imported deck's is.
+        Arc::make_mut(&mut session.doc_mut().unwrap().doc).header_footer.date_format = String::new();
+        let date_fields = |app: &SlideApp| -> Vec<String> {
+            app.session
+                .doc()
+                .unwrap()
+                .doc
+                .slides
+                .iter()
+                .flat_map(|sl| sl.shapes.iter())
+                .filter(|sh| sh.ph_type() == Some(deckcraft_model::PhType::Date))
+                .flat_map(|sh| sh.text.iter())
+                .flat_map(|t| t.paragraphs.iter())
+                .flat_map(|p| p.runs.iter())
+                .filter_map(|r| match &r.kind {
+                    deckcraft_model::text::RunKind::Field { field } => Some(field.clone()),
+                    _ => None,
+                })
+                .collect()
+        };
+        let mut harness = egui_kittest::Harness::builder().with_size(vec2(1200.0, 900.0)).build_ui_state(
+            |ui, app: &mut SlideApp| {
+                app.logic(ui.ctx());
+                app.ui(ui);
+            },
+            SlideApp::new(session, crate::Services::default()),
+        );
+        harness.run_steps(2);
+        assert_eq!(date_fields(harness.state()), vec!["datetime4".to_string()]);
+        harness.state_mut().run("app.dialog", json!({"id": "headerFooter"})).unwrap();
+        harness.run_steps(2);
+        harness.get_by_label("Apply").click_accesskit();
+        harness.run_steps(3);
+        assert_eq!(date_fields(harness.state()), vec!["datetime4".to_string()], "the dialog left the field's format alone");
+    }
+
     /// The Export… dialog (Animated GIF) shows the GIF options and exports through `file.export` (#171).
     #[test]
     fn export_dialog_writes_an_animated_gif() {