Default width of a Label in a Horizontal

Regarding the following script:

from textual.app import App, ComposeResult
from textual.containers import Horizontal
from textual.widgets import Label

class TestApp(App):

    CSS = """
    #left {
        background: blue;
        dock: left;
    }
    #right {
        background: darkred;
        dock: right;
    }
    """

    def compose(self) -> ComposeResult:
        left = Label('Left', id='left')
        right = Label('Right', id='right')
        yield Horizontal(left, right)

app = TestApp()
app.run()

When run, the above displays only the #right label. This is somewhat unexpected, and adding width: 1fr to both #left and #right doesn’t change the result, which I also found surprising. Adding width: 1fr to #left and width: auto to #right shows both labels, with #left taking up all of the space not used by #right.

It seems I’m not understanding something about how Textual CSS and/or Horizontal layouts work - can someone please help me out? I couldn’t see any CSS rules that imply that a Label would occupy 100% of a Horizontal, which seems to be the case. Also, also why doesn’t width: 1fr size the labels equally?