[Python] tkinter layout problem, pack does not place the elements as expected

I want to have three elements. The first two aligned left and the third below spanning the remaining space in x and y.

This code however does not realized that import ttkbootstrap as tb

class App(tb.Window):
def __init__(self):
# Initialize window with superhero theme
super().__init__(themename="superhero")

# Configure the root window
self.title('test layout')
self.geometry('800x600')

tb.Button(self, text='first').pack(padx=10, pady=10, side = tb.LEFT)
tb.Button(self, text='second').pack(padx=10, pady=10, side = tb.LEFT)
tb.Label(self, text="filled", borderwidth=2, relief="groove").pack(expand=True, side=tb.BOTTOM)


if __name__ == "__main__":
app = App()
app.mainloop()


This looks like

What is the typical solution?

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