Have you checked closed issues? (https://github.com/Textualize/textual/issues?q=is%3Aissue+is%3Aclosed)
Yes.
Have you checked against the most recent version of Textual? (https://pypi.org/search/?q=textual)
Yes
In short, if you have a class that extends Widget:
class MyWidget(Widget):
def compose(self) -> ComposeResult:
yield Button("Hello, World!")
with TabbedContent("MyWidget"):
yield MyWidget() # MyWidget is not displayed in the MyWidget tab.