Size Markdown table keylines from the same geometry as their cells

src/textual/layout.py

@@ -336,7 +336,10 @@ def render_keyline(self, container: Widget) -> StripRenderable:
         Returns:
             A renderable to draw the keylines.
         """
-        width, height = container.outer_size
+        # Keylines and their child rectangles belong to the same geometry.
+        # A complete subtree capture admits offscreen boxes before those
+        # widgets receive resize notifications; outer_size is that older value.
+        width, height = container.region.size
         canvas = Canvas(width, height)
 
         line_style, keyline_color = container.styles.keyline

tests/test_viewport_geometry_targets.py

@@ -8,6 +8,46 @@
 from textual.widgets import Static
 
 
+async def test_complete_markdown_capture_keeps_offscreen_table_keylines():
+    from textual.widgets import Markdown
+    from textual.widgets._markdown import MarkdownTableContent
+
+    source = "\n\n".join(
+        f"| key | value |\n| --- | --- |\n| TABLE_{index:02} | owned source |"
+        for index in range(12)
+    )
+    body = Markdown(source)
+
+    class TableScreen(Screen):
+        def _use_viewport_layout(self):
+            return True
+
+        def compose(self):
+            with VerticalScroll():
+                yield body
+
+    app = App()
+    async with app.run_test(size=(60, 12)) as pilot:
+        screen = TableScreen()
+        await app.push_screen(screen)
+        await pilot.pause()
+        compositor = screen._compositor
+        tables = list(body.query(MarkdownTableContent))
+        assert len(tables) == 12
+        assert tables[-1].outer_size.height == 0
+        assert tables[-1] not in compositor._published_map
+        _, placement = next(compositor.published_geometry((body,)))
+        published = compositor._full_map, compositor._visible_map
+        _, strips = compositor.render_subtree_strips(body, placement)
+        rows = [strip.text for strip in strips]
+        for index in range(12):
+            row = next(row for row in rows if f"TABLE_{index:02}" in row)
+            assert "│" in row, f"Offscreen table {index} lost its captured keyline"
+        assert compositor._full_map is published[0]
+        assert compositor._visible_map is published[1]
+        assert tables[-1].outer_size.height == 0
+
+
 class TargetedScreen(Screen):
     CSS = "VerticalScroll { width: 1fr; } VerticalGroup, Static { height: auto; }"
     targets = ()