Added some unit tests for the grid

docs/Datagrid Tests.md

@@ -0,0 +1,118 @@
+# DataGrid Tests — Backlog
+
+Source file: `tests/controls/test_datagrid.py`
+
+Legend: ✅ Done — ⬜ Pending
+
+---
+
+## TestDataGridBehaviour
+
+### Edition flow
+
+| # | Status | Test                                                      | Description                                              |
+|---|--------|-----------------------------------------------------------|----------------------------------------------------------|
+| 1 | ⬜      | `test_i_can_convert_edition_value_for_number`             | `"3.14"` → `float`, `"5"` → `int`                        |
+| 2 | ⬜      | `test_i_can_convert_edition_value_for_bool`               | `"true"`, `"1"`, `"yes"` → `True`; others → `False`      |
+| 3 | ⬜      | `test_i_can_convert_edition_value_for_text`               | String value returned unchanged                          |
+| 4 | ⬜      | `test_i_can_handle_start_edition`                         | Sets `edition.under_edition` and returns a cell render   |
+| 5 | ⬜      | `test_i_cannot_handle_start_edition_when_already_editing` | Second call while `under_edition` is set is a no-op      |
+| 6 | ⬜      | `test_i_can_handle_save_edition`                          | Writes value to data service and clears `under_edition`  |
+| 7 | ⬜      | `test_i_cannot_handle_save_edition_when_not_editing`      | Returns partial render without touching the data service |
+
+### Column management
+
+| #  | Status | Test                                                    | Description                                                    |
+|----|--------|---------------------------------------------------------|----------------------------------------------------------------|
+| 8  | ⬜      | `test_i_can_add_new_column`                             | Appends column to `_state.columns` and `_columns`              |
+| 9  | ⬜      | `test_i_can_handle_columns_reorder`                     | Reorders `_state.columns` according to provided list           |
+| 10 | ⬜      | `test_i_can_handle_columns_reorder_ignores_unknown_ids` | Unknown IDs skipped; known columns not in list appended at end |
+
+### Mouse selection
+
+| #  | Status | Test                                            | Description                                                   |
+|----|--------|-------------------------------------------------|---------------------------------------------------------------|
+| 11 | ⬜      | `test_i_can_on_mouse_selection_sets_range`      | Sets `extra_selected` with `("range", ...)` from two cell IDs |
+| 12 | ⬜      | `test_i_cannot_on_mouse_selection_when_outside` | `is_inside=False` leaves `extra_selected` unchanged           |
+
+### Key pressed
+
+| #  | Status | Test                                             | Description                                                                                  |
+|----|--------|--------------------------------------------------|----------------------------------------------------------------------------------------------|
+| 13 | ⬜      | `test_i_can_on_key_pressed_enter_starts_edition` | `enter` on selected cell enters edition when `enable_edition=True` and nothing under edition |
+
+### Click
+
+| #  | Status | Test                                              | Description                                                     |
+|----|--------|---------------------------------------------------|-----------------------------------------------------------------|
+| 14 | ⬜      | `test_i_can_on_click_second_click_enters_edition` | Second click on already-selected cell triggers `_enter_edition` |
+
+### Filtering / sorting
+
+| #  | Status | Test                                       | Description                                                                         |
+|----|--------|--------------------------------------------|-------------------------------------------------------------------------------------|
+| 15 | ⬜      | `test_i_can_filter_grid`                   | `filter()` updates `_state.filtered`; filtered DataFrame excludes non-matching rows |
+| 16 | ⬜      | `test_i_can_apply_sort`                    | `_apply_sort` returns rows in correct order when a sort definition is present       |
+| 17 | ⬜      | `test_i_can_apply_filter_by_column_values` | Column filter (non-FILTER_INPUT) keeps only matching rows                           |
+
+### Format rules priority
+
+| #  | Status | Test                                                                 | Description                                                      |
+|----|--------|----------------------------------------------------------------------|------------------------------------------------------------------|
+| 18 | ⬜      | `test_i_can_get_format_rules_cell_level_takes_priority`              | Cell format overrides row, column and table format               |
+| 19 | ⬜      | `test_i_can_get_format_rules_row_level_takes_priority_over_column`   | Row format overrides column and table when no cell format        |
+| 20 | ⬜      | `test_i_can_get_format_rules_column_level_takes_priority_over_table` | Column format overrides table when no cell or row format         |
+| 21 | ⬜      | `test_i_can_get_format_rules_falls_back_to_table_format`             | Table format returned when no cell, row or column format defined |
+
+---
+
+## TestDataGridRender
+
+### Table structure
+
+| #  | Status | Test                                     | Description                                                                               |
+|----|--------|------------------------------------------|-------------------------------------------------------------------------------------------|
+| 22 | ✅      | `test_i_can_render_table_wrapper`        | ID `tw_{id}`, class `dt2-table-wrapper`, 3 sections: selection manager, table, scrollbars |
+| 23 | ✅      | `test_i_can_render_table`                | ID `t_{id}`, class `dt2-table`, 3 containers: header, body wrapper, footer                |
+| 24 | ✅      | `test_i_can_render_table_has_scrollbars` | Scrollbars overlay contains vertical and horizontal tracks                                |
+
+### render_partial fragments
+
+| #  | Status | Test                                              | Description                                                                          |
+|----|--------|---------------------------------------------------|--------------------------------------------------------------------------------------|
+| 25 | ✅      | `test_i_can_render_partial_body`                  | Returns `(selection_manager, body_wrapper)` — body wrapper has `hx-on::after-settle` |
+| 26 | ✅      | `test_i_can_render_partial_table`                 | Returns `(selection_manager, table)` — table has `hx-on::after-settle`               |
+| 27 | ✅      | `test_i_can_render_partial_header`                | Returns header with `hx-on::after-settle` containing `setColumnWidth`                |
+| 28 | ✅      | `test_i_can_render_partial_cell_by_pos`           | Returns `(selection_manager, cell)` for a specific `(col, row)` position             |
+| 29 | ✅      | `test_i_can_render_partial_cell_with_no_position` | Returns only `(selection_manager,)` when no `pos` or `cell_id` given                 |
+
+### Edition cell
+
+| #  | Status | Test                                          | Description                                                                                              |
+|----|--------|-----------------------------------------------|----------------------------------------------------------------------------------------------------------|
+| 30 | ⬜      | `test_i_can_render_body_cell_in_edition_mode` | When `edition.under_edition` matches, `mk_body_cell` returns an input cell with class `dt2-cell-edition` |
+
+### Cell content — search highlighting
+
+| #  | Status | Test                                                                        | Description                                                     |
+|----|--------|-----------------------------------------------------------------------------|-----------------------------------------------------------------|
+| 31 | ⬜      | `test_i_can_render_body_cell_content_with_search_highlight`                 | Matching keyword produces a `Span` with class `dt2-highlight-1` |
+| 32 | ⬜      | `test_i_can_render_body_cell_content_with_no_highlight_when_keyword_absent` | Non-matching keyword produces no `dt2-highlight-1` span         |
+
+### Footer
+
+| #  | Status | Test                                                      | Description                                                              |
+|----|--------|-----------------------------------------------------------|--------------------------------------------------------------------------|
+| 33 | ⬜      | `test_i_can_render_footers_wrapper`                       | `mk_footers` renders with ID `tf_{id}` and class `dt2-footer`            |
+| 34 | ⬜      | `test_i_can_render_aggregation_cell_sum`                  | `mk_aggregation_cell` with `FooterAggregation.Sum` renders the sum value |
+| 35 | ⬜      | `test_i_cannot_render_aggregation_cell_for_hidden_column` | Hidden column returns `Div(cls="dt2-col-hidden")`                        |
+
+---
+
+## Summary
+
+| Class                   | Total  | ✅ Done | ⬜ Pending |
+|-------------------------|--------|--------|-----------|
+| `TestDataGridBehaviour` | 21     | 0      | 21        |
+| `TestDataGridRender`    | 14     | 8      | 6         |
+| **Total**               | **35** | **8**  | **27**    |

tests/controls/test_datagrid.py

@@ -79,14 +79,6 @@ def datagrid_with_full_data(datagrids_manager):
   return dg
 
 
-@pytest.fixture
-def datagrid_no_edition(datagrid_with_data):
-  """DataGrid with edition disabled (no RowSelection column, no add-column button)."""
-  dg = datagrid_with_data
-  dg._settings.enable_edition = False
-  dg._init_columns()
-  return dg
-
 
 class TestDataGridBehaviour:
   def test_i_can_create_empty_datagrid(self, datagrids_manager):
@@ -150,24 +142,20 @@ class TestDataGridBehaviour:
   # Element ID Parsing
   # ------------------------------------------------------------------
   
-  def test_i_can_get_pos_from_cell_element_id(self, datagrid):
+  @pytest.mark.parametrize("element_id_template, expected", [
-    """Test that _get_pos_from_element_id correctly parses (col, row) from a cell ID.
+      ("tcell_{id}-3-7", (3, 7)),
+      ("trow_{id}-5",    None),
+      (None,             None),
+  ])
+  def test_i_can_get_pos_from_element_id(self, datagrid, element_id_template, expected):
+    """Test that _get_pos_from_element_id returns the correct (col, row) position or None.
 
-    The position tuple (col, row) is used for cell navigation and selection
+    - Cell IDs ('tcell_…') carry (col, row) indices required for cell navigation.
-    state tracking. Correct parsing is required for keyboard navigation and
+    - Row IDs ('trow_…') have no cell position; None signals no cell can be derived.
-    mouse selection to target the right cell.
+    - None input is a safe no-op; callers must handle it without raising.
     """
-    element_id = f"tcell_{datagrid._id}-3-7"
+    element_id = element_id_template.format(id=datagrid._id) if element_id_template else None
-    assert datagrid._get_pos_from_element_id(element_id) == (3, 7)
+    assert datagrid._get_pos_from_element_id(element_id) == expected
-  
-  def test_i_can_get_pos_returns_none_for_non_cell_id(self, datagrid):
-    """Test that _get_pos_from_element_id returns None for row IDs and None input.
-
-    Row and column IDs don't carry a (col, row) position. Returning None
-    signals that no cell-level position can be derived.
-    """
-    assert datagrid._get_pos_from_element_id(f"trow_{datagrid._id}-5") is None
-    assert datagrid._get_pos_from_element_id(None) is None
   
   # ------------------------------------------------------------------
   # Static ID Conversions
@@ -488,60 +476,28 @@ class TestDataGridRender:
     )
     assert matches(html, expected)
   
-  def test_i_can_render_extra_selected_row(self, datagrid):
+  @pytest.mark.parametrize("sel_type, element_id_template", [
-    """Test that a row extra-selection entry renders as a Div with selection_type='row'.
+      ("row",    "trow_{id}-3"),
+      ("column", "tcol_{id}-2"),
+      ("range",  (0, 0, 2, 2)),
+  ])
+  def test_i_can_render_extra_selected_entry(self, datagrid, sel_type, element_id_template):
+    """Test that each extra-selection type renders as a child Div with the correct attributes.
 
     Why these elements matter:
-    - selection_type='row': JS applies the row-stripe highlight to the entire row
+    - selection_type: tells JS which highlight strategy to apply (row stripe,
-    - element_id: the DOM ID of the row element that JS will highlight
+      column stripe, or range rectangle)
+    - element_id: the DOM target JS will highlight; strings are used as-is,
+      tuples (range bounds) are stringified so JS can parse the coordinates
     """
     dg = datagrid
-    row_element_id = f"trow_{dg._id}-3"
+    element_id = element_id_template.format(id=dg._id) if isinstance(element_id_template, str) else element_id_template
-    dg._state.selection.extra_selected.append(("row", row_element_id))
+    dg._state.selection.extra_selected.append((sel_type, element_id))
 
     html = dg.mk_selection_manager()
 
     expected = Div(
-      Div(selection_type="row", element_id=row_element_id),
+      Div(selection_type=sel_type, element_id=f"{element_id}"),
-      id=f"tsm_{dg._id}",
-    )
-    assert matches(html, expected)
-  
-  def test_i_can_render_extra_selected_column(self, datagrid):
-    """Test that a column extra-selection entry renders as a Div with selection_type='column'.
-
-    Why these elements matter:
-    - selection_type='column': JS applies the column-stripe highlight to the entire column
-    - element_id: the DOM ID of the column header element that JS will highlight
-    """
-    dg = datagrid
-    col_element_id = f"tcol_{dg._id}-2"
-    dg._state.selection.extra_selected.append(("column", col_element_id))
-    
-    html = dg.mk_selection_manager()
-    
-    expected = Div(
-      Div(selection_type="column", element_id=col_element_id),
-      id=f"tsm_{dg._id}",
-    )
-    assert matches(html, expected)
-  
-  def test_i_can_render_extra_selected_range(self, datagrid):
-    """Test that a range extra-selection entry renders with the tuple stringified as element_id.
-
-    Why these elements matter:
-    - selection_type='range': JS draws a rectangular highlight over the cell region
-    - element_id=str(tuple): the range bounds (min_col, min_row, max_col, max_row)
-      are passed as a string; JS parses this to locate all cells in the rectangle
-    """
-    dg = datagrid
-    range_bounds = (0, 0, 2, 2)
-    dg._state.selection.extra_selected.append(("range", range_bounds))
-    
-    html = dg.mk_selection_manager()
-    
-    expected = Div(
-      Div(selection_type="range", element_id=f"{range_bounds}"),
       id=f"tsm_{dg._id}",
     )
     assert matches(html, expected)
@@ -612,60 +568,34 @@ class TestDataGridRender:
     col_headers = find(html, Div(cls=Contains("dt2-cell", "dt2-resizable")))
     assert len(col_headers) == 3, "Should have one resizable header cell per visible data column"
   
-  def test_i_can_render_row_selection_header_in_edition_mode(self, datagrid_with_data):
+  @pytest.mark.parametrize("css_cls, edition_enabled, expected_count", [
-    """Test that a RowSelection header cell is rendered when edition mode is enabled.
+      ("dt2-row-selection", True,  1),
+      ("dt2-row-selection", False, 0),
+      ("dt2-add-column",    True,  1),
+      ("dt2-add-column",    False, 0),
+  ])
+  def test_i_can_render_header_edition_elements_visibility(
+      self, datagrid_with_data, css_cls, edition_enabled, expected_count):
+    """Test that edition-specific header elements are present only when edition is enabled.
 
     Why these elements matter:
-    - dt2-row-selection: the selection checkbox column is only meaningful in edition
+    - dt2-row-selection: the checkbox column is only meaningful in edition mode;
-      mode where rows can be individually selected for bulk operations; JS uses this
+      rendering it in read-only mode would create an orphan misaligned column
-      cell to anchor the row-selection toggle handler
+    - dt2-add-column: the '+' icon exposes mutation UI; it must be hidden in
-    - exactly 1 cell: a second dt2-row-selection would double the checkbox column
+      read-only grids to prevent users from adding columns unintentionally
+    - expected_count 1 vs 0: exactly one element when enabled, none when disabled,
+      prevents both missing controls and duplicated ones
     """
     dg = datagrid_with_data
+    dg._settings.enable_edition = edition_enabled
+    dg._init_columns()
     html = dg.mk_headers()
 
-    row_sel_cells = find(html, Div(cls=Contains("dt2-row-selection")))
+    elements = find(html, Div(cls=Contains(css_cls)))
-    assert len(row_sel_cells) == 1, "Edition mode must render exactly one row-selection header cell"
+    assert len(elements) == expected_count, (
-  
+        f"{'Edition' if edition_enabled else 'Read-only'} mode must render "
-  def test_i_cannot_render_row_selection_header_without_edition_mode(self, datagrid_no_edition):
+        f"exactly {expected_count} '{css_cls}' element(s)"
-    """Test that no RowSelection header cell is rendered when edition mode is disabled.
+    )
-
-    Why this matters:
-    - Without edition, there is no row selection column in _columns; rendering one
-      would create an orphan cell misaligned with the body rows
-    """
-    dg = datagrid_no_edition
-    html = dg.mk_headers()
-    
-    row_sel_cells = find(html, Div(cls=Contains("dt2-row-selection")))
-    assert len(row_sel_cells) == 0, "Without edition mode, no row-selection header cell should be rendered"
-  
-  def test_i_can_render_add_column_button_in_edition_mode(self, datagrid_with_data):
-    """Test that the add-column button is appended to the header in edition mode.
-
-    Why this element matters:
-    - dt2-add-column: the '+' icon at the end of the header lets users add new
-      columns interactively; it must be present in edition mode and absent otherwise
-      to avoid exposing mutation UI in read-only grids
-    """
-    dg = datagrid_with_data
-    html = dg.mk_headers()
-    
-    add_col_cells = find(html, Div(cls=Contains("dt2-add-column")))
-    assert len(add_col_cells) == 1, "Edition mode must render exactly one add-column button"
-  
-  def test_i_cannot_render_add_column_button_without_edition_mode(self, datagrid_no_edition):
-    """Test that no add-column button is rendered when edition mode is disabled.
-
-    Why this matters:
-    - Read-only grids must not expose mutation controls; the absence of dt2-add-column
-      guarantees that the JS handler for toggling the column editor is never reachable
-    """
-    dg = datagrid_no_edition
-    html = dg.mk_headers()
-    
-    add_col_cells = find(html, Div(cls=Contains("dt2-add-column")))
-    assert len(add_col_cells) == 0, "Without edition mode, no add-column button should be rendered"
   
   def test_i_can_render_headers_in_column_order(self, datagrid_with_data):
     """Test that resizable header cells appear in the same order as self._columns.
@@ -787,6 +717,209 @@ class TestDataGridRender:
     ))
     assert len(handles) == 3, "Each data column must have exactly one resize handle with correct command IDs"
   
+  # ------------------------------------------------------------------
+  # Table structure
+  # ------------------------------------------------------------------
+
+  def test_i_can_render_table_wrapper(self, datagrid_with_data):
+    """Test that mk_table_wrapper renders with correct ID, class and 3 main sections.
+
+    Why these elements matter:
+    - id=tw_{id}: used by JS to position custom scrollbars over the table
+    - cls Contains 'dt2-table-wrapper': CSS hook for relative positioning that lets
+      the scrollbars overlay use absolute coordinates over the table
+    - tsm_{id}: selection manager lives inside the wrapper so it survives partial
+      re-renders that target the wrapper
+    - t_{id}: the table with header, body and footer
+    - dt2-scrollbars: custom scrollbar overlay (structure tested separately)
+    """
+    dg = datagrid_with_data
+    html = dg.mk_table_wrapper()
+    expected = Div(
+      Div(id=f"tsm_{dg._id}"),                 # selection manager
+      Div(id=f"t_{dg._id}"),                    # table
+      Div(cls=Contains("dt2-scrollbars")),       # scrollbars overlay
+      id=f"tw_{dg._id}",
+      cls=Contains("dt2-table-wrapper"),
+    )
+    assert matches(html, expected)
+
+  def test_i_can_render_table(self, datagrid_with_data):
+    """Test that mk_table renders with correct ID, class and 3 container sections.
+
+    Why these elements matter:
+    - id=t_{id}: targeted by on_column_changed and render_partial('table') swaps
+    - cls Contains 'dt2-table': CSS grid container that aligns header, body and
+      footer columns
+    - dt2-header-container: wraps the header row with no-scroll behaviour
+    - tb_{id}: body wrapper, targeted by get_page for lazy-load row appends and by
+      render_partial('body') for full body swaps on filter/sort
+    - dt2-footer-container: wraps the aggregation footer with no-scroll behaviour
+    """
+    dg = datagrid_with_data
+    html = dg.mk_table()
+    expected = Div(
+      Div(cls=Contains("dt2-header-container")),  # header container
+      Div(id=f"tb_{dg._id}"),                     # body wrapper
+      Div(cls=Contains("dt2-footer-container")),  # footer container
+      id=f"t_{dg._id}",
+      cls=Contains("dt2-table"),
+    )
+    assert matches(html, expected)
+
+  def test_i_can_render_table_has_scrollbars(self, datagrid_with_data):
+    """Test that the scrollbars overlay contains both vertical and horizontal tracks.
+
+    Why these elements matter:
+    - dt2-scrollbars-vertical-wrapper / dt2-scrollbars-horizontal-wrapper: JS resizes
+      these wrappers to match the live table dimensions on each render
+    - dt2-scrollbars-vertical / dt2-scrollbars-horizontal: the visible scrollbar
+      thumbs that JS moves on scroll; missing either disables that scroll axis
+    """
+    dg = datagrid_with_data
+    html = dg.mk_table_wrapper()
+
+    # Step 1: Find and validate the vertical scrollbar wrapper
+    vertical = find_one(html, Div(cls=Contains("dt2-scrollbars-vertical-wrapper")))
+    assert matches(vertical, Div(
+      Div(cls=Contains("dt2-scrollbars-vertical")),
+      cls=Contains("dt2-scrollbars-vertical-wrapper"),
+    ))
+
+    # Step 2: Find and validate the horizontal scrollbar wrapper
+    horizontal = find_one(html, Div(cls=Contains("dt2-scrollbars-horizontal-wrapper")))
+    assert matches(horizontal, Div(
+      Div(cls=Contains("dt2-scrollbars-horizontal")),
+      cls=Contains("dt2-scrollbars-horizontal-wrapper"),
+    ))
+
+  # ------------------------------------------------------------------
+  # render_partial fragments
+  # ------------------------------------------------------------------
+
+  def test_i_can_render_partial_body(self, datagrid_with_data):
+    """Test that render_partial('body') returns (selection_manager, body_wrapper).
+
+    Why these elements matter:
+    - 2 elements: both the body and the selection manager are sent back together
+      so the cell highlight is updated in the same response as the body swap
+    - tsm_{id}: refreshes the cell highlight after the body is replaced
+    - tb_{id}: the HTMX target for filter and sort re-renders
+    - hx-on::after-settle Contains 'initDataGrid': re-initialises JS scroll and
+      resize logic after the new body is inserted into the DOM
+    """
+    dg = datagrid_with_data
+    result = dg.render_partial("body")
+
+    # Step 1: Verify tuple length
+    assert len(result) == 2, "render_partial('body') must return (selection_manager, body_wrapper)"
+
+    # Step 2: Verify selection manager
+    assert matches(result[0], Div(id=f"tsm_{dg._id}"))
+
+    # Step 3: Verify body wrapper ID, class and after-settle attribute
+    assert matches(result[1], Div(id=f"tb_{dg._id}", cls=Contains("dt2-body-container")))
+    assert "initDataGrid" in result[1].attrs.get("hx-on::after-settle", ""), (
+        "Body wrapper must carry hx-on::after-settle with initDataGrid to re-init JS after swap"
+    )
+
+  def test_i_can_render_partial_table(self, datagrid_with_data):
+    """Test that render_partial('table') returns (selection_manager, table).
+
+    Why these elements matter:
+    - 2 elements: body and selection manager are sent back together so the cell
+      highlight is updated in the same response as the table swap
+    - t_{id}: full table swap used by on_column_changed when columns are added,
+      hidden, or reordered; an incorrect ID would leave the old table in the DOM
+    - hx-on::after-settle Contains 'initDataGrid': re-initialises column resize
+      and drag-and-drop after the new table structure is inserted
+    """
+    dg = datagrid_with_data
+    result = dg.render_partial("table")
+
+    # Step 1: Verify tuple length
+    assert len(result) == 2, "render_partial('table') must return (selection_manager, table)"
+
+    # Step 2: Verify selection manager
+    assert matches(result[0], Div(id=f"tsm_{dg._id}"))
+
+    # Step 3: Verify table ID, class and after-settle attribute
+    assert matches(result[1], Div(id=f"t_{dg._id}", cls=Contains("dt2-table")))
+    assert "initDataGrid" in result[1].attrs.get("hx-on::after-settle", ""), (
+        "Table must carry hx-on::after-settle with initDataGrid to re-init JS after column swap"
+    )
+
+  def test_i_can_render_partial_header(self, datagrid_with_data):
+    """Test that render_partial('header') returns a single header element with setColumnWidth.
+
+    Why these elements matter:
+    - not a tuple: header swaps are triggered by reset_column_width which uses a
+      direct HTMX target (#th_{id}); returning a tuple would break the swap
+    - th_{id}: the HTMX target for the header swap after auto-size
+    - hx-on::after-settle Contains 'setColumnWidth': applies the new pixel width
+      to all body cells via JS after the header is swapped in
+    - col_id in after-settle: JS needs the column ID to target the correct cells
+    """
+    dg = datagrid_with_data
+    col_id = dg._state.columns[0].col_id
+    result = dg.render_partial("header", col_id=col_id, optimal_width=200)
+
+    # Step 1: Verify it is a single element, not a tuple
+    assert not isinstance(result, tuple), "render_partial('header') must return a single element"
+
+    # Step 2: Verify header ID and class
+    assert matches(result, Div(id=f"th_{dg._id}", cls=Contains("dt2-header")))
+
+    # Step 3: Verify after-settle contains setColumnWidth and the column ID
+    after_settle = result.attrs.get("hx-on::after-settle", "")
+    assert "setColumnWidth" in after_settle, (
+        "Header must carry hx-on::after-settle with setColumnWidth to resize body cells"
+    )
+    assert col_id in after_settle, (
+        "hx-on::after-settle must include the column ID so JS targets the correct column"
+    )
+
+  def test_i_can_render_partial_cell_by_pos(self, datagrid_with_data):
+    """Test that render_partial('cell', pos=...) returns (selection_manager, cell).
+
+    Why these elements matter:
+    - 2 elements: cell content and selection manager are sent back together so
+      the focus highlight is updated in the same response as the cell swap
+    - tsm_{id}: refreshes the focus highlight after the cell is replaced
+    - tcell_{id}-{col}-{row}: the HTMX swap target for individual cell updates
+      (edition entry/exit); an incorrect ID leaves the old cell content in the DOM
+    """
+    dg = datagrid_with_data
+    name_col = next(c for c in dg._columns if c.title == "name")
+    col_pos = dg._columns.index(name_col)
+    result = dg.render_partial("cell", pos=(col_pos, 0))
+
+    # Step 1: Verify tuple length
+    assert len(result) == 2, "render_partial('cell', pos=...) must return (selection_manager, cell)"
+
+    # Step 2: Verify selection manager
+    assert matches(result[0], Div(id=f"tsm_{dg._id}"))
+
+    # Step 3: Verify cell ID and class
+    assert matches(result[1], Div(
+      id=f"tcell_{dg._id}-{col_pos}-0",
+      cls=Contains("dt2-cell"),
+    ))
+
+  def test_i_can_render_partial_cell_with_no_position(self, datagrid_with_data):
+    """Test that render_partial() with no position returns only (selection_manager,).
+
+    Why this matters:
+    - 1 element only: when no valid cell position can be resolved, only the
+      selection manager is returned to refresh the highlight state
+    - no cell element: no position means no cell to update in the DOM
+    """
+    dg = datagrid_with_data
+    result = dg.render_partial()
+
+    assert len(result) == 1, "render_partial() with no position must return only (selection_manager,)"
+    assert matches(result[0], Div(id=f"tsm_{dg._id}"))
+
   # ------------------------------------------------------------------
   # Body
   # ------------------------------------------------------------------