fix(canvas): seed an empty paragraph when pouring text into a fresh cell

crates/paged-canvas/src/mutate.rs

@@ -193,6 +193,16 @@ fn apply_insert_text(
         });
     }
 
+    // A freshly-created table cell (and any other empty paragraph stream)
+    // carries ZERO paragraphs. `locate` pins an empty stream to
+    // `paragraph_idx 0` (via `saturating_sub(1)`), and `insert_one_segment`
+    // would then index an empty slice — the `index out of bounds: len is 0
+    // but the index is 0` panic the sheet cell-pour hit. Seed one empty
+    // paragraph so the first write into a new cell has a stream to land in.
+    if paragraphs.is_empty() {
+        paragraphs.push(paged_parse::Paragraph::default());
+    }
+
     // Phase 3 Gap-D: split text on `\n`. Each segment becomes a
     // contiguous insert within a (possibly new) paragraph. Multiple
     // `\n`s in the source split into multiple paragraphs.

crates/paged-canvas/tests/cell_text.rs

@@ -244,6 +244,45 @@ fn insert_text_into_cell_then_read_back() {
     }
 }
 
+#[test]
+fn pour_text_into_a_freshly_inserted_empty_table_cell_does_not_panic() {
+    // Regression (the sheet cell-pour panic): a cell created by `insertTable`
+    // carries ZERO paragraphs. The first text pour into it located the end of
+    // an empty stream and indexed `paragraphs[0]` on an empty slice —
+    // `index out of bounds: the len is 0 but the index is 0` in
+    // `insert_one_segment`. `apply_insert_text` now seeds one empty paragraph
+    // when the cell stream is empty, so the first write lands cleanly.
+    let mut model = load_model();
+    let out = model
+        .apply_mutation(&Mutation::InsertTable {
+            story_id: "u10".into(),
+            rows: 2,
+            cols: 2,
+            header_rows: 0,
+            footer_rows: 0,
+            column_widths: vec![],
+            row_heights: vec![],
+        })
+        .expect("insert a 2x2 table into the story");
+    let table_id = match out.created_id {
+        Some(paged_canvas::ElementId::Table { table_id, .. }) => table_id,
+        other => panic!("insertTable must report a Table createdId, got {other:?}"),
+    };
+    // The brand-new cell (0,0) has no paragraphs; pouring text MUST NOT panic.
+    model
+        .apply_mutation(&Mutation::InsertText {
+            story_id: "u10".into(),
+            offset: 0,
+            text: "Hi".into(),
+            cell: Some(TextCellAddr {
+                table_id,
+                row: 0,
+                col: 0,
+            }),
+        })
+        .expect("pour text into the fresh empty cell");
+}
+
 #[test]
 fn insert_into_cell_undo_restores_exact_prior_content() {
     let mut model = load_model();

crates/paged-canvas/tests/tablecell_wire.rs

@@ -0,0 +1,38 @@
+// Wire pin: the EXACT op shapes paged.sheet's lowering emits for a
+// native-table cell pour must deserialize as the wire `Mutation`. (A bundle
+// bug once nested the whole Table id as `table_id`, which the engine rejected
+// with "invalid type: map, expected a string" — this pins the correct shapes
+// so a regression fails loudly here.)
+
+use paged_canvas::Mutation;
+
+fn try_de(label: &str, json: &str) {
+    match serde_json::from_str::<Mutation>(json) {
+        Ok(_) => println!("OK   {label}"),
+        Err(e) => println!("FAIL {label}: {e}"),
+    }
+}
+
+#[test]
+fn sheet_cell_pour_ops_deserialize() {
+    // 1. cell fill — setElementProperty on a tableCell, colorRef string value.
+    try_de(
+        "cellFillColor",
+        r#"{"op":"setElementProperty","args":{"elementId":{"kind":"tableCell","id":{"story_id":"u1","table_id":"t1","row":0,"col":0}},"path":"cellFillColor","value":{"type":"colorRef","value":"Color/Black"}}}"#,
+    );
+    // 2. edge stroke — setElementProperty on a tableCell, length value.
+    try_de(
+        "cellTopEdgeStrokeWeight",
+        r#"{"op":"setElementProperty","args":{"elementId":{"kind":"tableCell","id":{"story_id":"u1","table_id":"t1","row":0,"col":0}},"path":"cellTopEdgeStrokeWeight","value":{"type":"length","value":0.5}}}"#,
+    );
+    // 3. text pour — insertText with the cell qualifier (TextCellAddr).
+    try_de(
+        "insertText.cell",
+        r#"{"op":"insertText","args":{"storyId":"u1","offset":0,"text":"hi","cell":{"tableId":"t1","row":0,"col":0}}}"#,
+    );
+    // 4. span — setCellSpan.
+    try_de(
+        "setCellSpan",
+        r#"{"op":"setCellSpan","args":{"storyId":"u1","tableId":"t1","row":0,"col":0,"rowSpan":2,"columnSpan":1}}"#,
+    );
+}