.ai import: a file whose PDF part is an empty page opens from its editing data

Squashed from #4651 onto main.

crates/engine/src/cmd/fileio/load.rs

@@ -197,6 +197,13 @@ fn from_editing_data_alone(bytes: &[u8], opts: &LoadOptions) -> Option<Result<(D
     )
 }
 
+/// Does `doc` hold no art: only layers, with nothing in them?
+fn draws_nothing(doc: &Document) -> bool {
+    let mut art = false;
+    doc.walk(|n| art |= !n.is_layer());
+    !art
+}
+
 /// Is `e` the error of a file whose pages are only a placeholder
 /// ([`vectorcraft_pdf::PdfError::PlaceholderOnly`])?
 pub(crate) fn is_placeholder(e: &EngineError) -> bool {
@@ -272,8 +279,13 @@ pub fn load_with(name: &str, bytes: &[u8], opts: &LoadOptions) -> Result<Loaded>
             restore_or_import(editing, || match super::pdfimport::import(bytes, opts, format.id != "pdf") {
                 Ok(r) => {
                     // Pages of text alone: the editing copy alone where it reads, as for a file
-                    // saved without its PDF part (#1387: else the page, which is the art).
-                    let alone = r.only_text.then(|| from_editing_data_alone(bytes, opts)).flatten().and_then(|read| read.ok());
+                    // saved without its PDF part (#1387: else the page, which is the art). So too
+                    // pages that draw nothing at all: there is no page art to check the layers
+                    // against, and the empty page would lose all of it.
+                    let alone = (r.only_text || draws_nothing(&r.document))
+                        .then(|| from_editing_data_alone(bytes, opts))
+                        .flatten()
+                        .and_then(|read| read.ok());
                     let (doc, warnings, read) = match alone {
                         Some((doc, warnings)) => (doc, warnings, true),
                         None => through_editing_data(bytes, opts, r.document, r.warnings, r.user_unit),

crates/engine/src/cmd/fileio/tests_aiimport.rs

@@ -556,6 +556,26 @@ fn an_ai_file_whose_pdf_part_is_cropped_to_its_art_opens_from_its_layers() {
     assert!((b.x0 - 100.0).abs() < 0.01 && (b.y0 - 50.0).abs() < 0.01, "where the artboard has it: {b:?}");
 }
 
+#[test]
+fn an_ai_file_whose_pdf_part_draws_nothing_opens_from_its_editing_data() {
+    // A PDF-compatible `.ai` whose PDF part is a page with nothing on it (no notice text either),
+    // with or without the boxes that say where its art is: its editing data is the art.
+    for boxed in ["", "/ArtBox [0 0 200 100] /TrimBox [0 0 200 100]"] {
+        let mut s = Session::new();
+        let r = open(&mut s, "blank.ai", &ai::ai_with(&compressed(&ai::sample_data()), "", boxed), json!({})).unwrap();
+        assert_eq!(r["warnings"], json!([]), "{boxed:?}: {r}");
+        check_sample(&s);
+        // The PDF part alone, as asked: its empty page.
+        open(&mut s, "blank.ai", &ai::ai_with(&compressed(&ai::sample_data()), "", boxed), json!({"editingData": false})).unwrap();
+        let layers = &s.doc().unwrap().doc.layers;
+        assert!(layers.iter().all(|l| l.children().is_none_or(|c| c.is_empty())), "{boxed:?}");
+    }
+    // Editing data that can't be read leaves the empty page, as before.
+    let mut s = Session::new();
+    open(&mut s, "blank.ai", &ai::ai(b"%AI24_ZStandard_Data\0\0\0\0\0\0\0\0", ""), json!({})).unwrap();
+    assert_ne!(names(&s.doc().unwrap().doc.layers), ["Art", "Hidden"]);
+}
+
 #[test]
 fn editing_data_false_opens_only_what_the_page_prints() {
     let mut s = Session::new();