fix(tabbed content): prevent duplicate ids error

Fixes #5215 where removing
then adding a pane could crash with a `DuplicateIds` exception.

Rather than assigning the ID based on the *current* tab count, this adds
a `_cumulative_tab_count` to ensure added panes have a unique ID.

src/textual/widgets/_tabbed_content.py

@@ -332,6 +332,7 @@ def __init__(
         self.titles = [self.render_str(title) for title in titles]
         self._tab_content: list[Widget] = []
         self._initial = initial
+        self._cumulative_tab_count = 0
         super().__init__(name=name, id=id, classes=classes, disabled=disabled)
 
     @property
@@ -384,6 +385,8 @@ def compose(self) -> ComposeResult:
             for content in pane_content
         ]
 
+        self._cumulative_tab_count = len(tabs)
+
         # Yield the tabs, and ensure they're linked to this TabbedContent.
         # It's important to associate the Tabs with the TabbedContent, so that this
         # TabbedContent can determine whether a message received from a Tabs instance
@@ -419,12 +422,13 @@ def add_pane(
             Only one of `before` or `after` can be provided. If both are
             provided an exception is raised.
         """
+        self._cumulative_tab_count += 1
         if isinstance(before, TabPane):
             before = before.id
         if isinstance(after, TabPane):
             after = after.id
         tabs = self.get_child_by_type(ContentTabs)
-        pane = self._set_id(pane, tabs.tab_count + 1)
+        pane = self._set_id(pane, self._cumulative_tab_count)
         assert pane.id is not None
         pane.display = False
         return AwaitComplete(

tests/test_tabbed_content.py

@@ -914,3 +914,23 @@ def compose(self) -> ComposeResult:
         await pilot.pause()
         assert app.query_one("Tab#duplicate").disabled is True
         assert app.query_one("TabPane#duplicate").disabled is False
+
+
+async def test_remove_and_add_pane_no_duplicate_id_error():
+    """Ensure that removing then adding panes does not raise a `DuplicateIds` exception.
+
+    Regression test for https://github.com/Textualize/textual/issues/5215
+    """
+
+    class TabbedApp(App[None]):
+        def compose(self) -> ComposeResult:
+            with TabbedContent():
+                yield Label("tab-1")
+                yield Label("tab-2")
+
+    app = TabbedApp()
+    async with app.run_test() as pilot:
+        # If no exception is raised, the test will pass
+        tabbed_content = pilot.app.query_one(TabbedContent)
+        await tabbed_content.remove_pane("tab-1")
+        await tabbed_content.add_pane(TabPane("Tab 3", Label("tab-3")))