[Python] pandas filling value until conditions in met

I have the following dataframe:

df = pd.DataFrame({"A": ["sell", np.nan, np.nan, np.nan, np.nan, "buy", np.nan, np.nan, np.nan, np.nan],
"B": ["buy", "buy", "sell", "buy", "buy", np.nan, "buy", "buy", "buy", np.nan],
"C": ["sell", "sell", "sell", "sell", "buy", "sell", "sell", "buy", "buy", np.nan]},
index=pd.date_range("2025-05-22", periods=10, freq="15min"))


print df

A B C
2025-05-22 00:00:00 sell buy sell
2025-05-22 00:15:00 NaN buy sell
2025-05-22 00:30:00 NaN sell sell
2025-05-22 00:45:00 NaN buy sell
2025-05-22 01:00:00 NaN buy buy
2025-05-22 01:15:00 buy NaN sell
2025-05-22 01:30:00 NaN buy sell
2025-05-22 01:45:00 NaN buy buy
2025-05-22 02:00:00 NaN buy buy
2025-05-22 02:15:00 NaN NaN buy


I want to forward fill the NaN in "A" column until all 3 columns are equals. It should look like this

A B C
2025-05-22 00:00:00 sell buy sell
2025-05-22 00:15:00 sell buy sell
2025-05-22 00:30:00 sell sell sell
2025-05-22 00:45:00 NaN buy sell
2025-05-22 01:00:00 NaN buy buy
2025-05-22 01:15:00 buy NaN sell
2025-05-22 01:30:00 buy buy sell
2025-05-22 01:45:00 buy buy buy
2025-05-22 02:00:00 NaN buy buy
2025-05-22 02:15:00 NaN NaN buy


Any suggestion will be greatly appreciated

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