[BUG]: contours.size initialization does not apply the specified value, but updates via dropdown work correctly

[williamlus]

Description

When initializing a go.Contour trace with a specific contours.size value (e.g., 0.25), the setting is ignored on the initial render, and Plotly defaults to an automatically calculated contour step size.

Interestingly, if the exact same parameter ("contours.size") is later updated dynamically via a layout updatemenus dropdown using the "restyle" method, the chart correctly updates and respects the specified size.

Screenshots/Video

• Initial Render (Buggy): The contours are spaced at default intervals despite contours.size=0.25 being passed to the constructor.

• After Selecting Dropdown (Expected): Selecting "Contour Size: 0.25" from the dropdown forces the chart to restyle, properly displaying the dense 0.25-step contour lines.

Steps to reproduce

1. Run the provided Python snippet using plotly.py.
2. Observe the initial plot render: notice that the contour lines do not match a step size of 0.25.
3. Click the dropdown menu at the top left of the figure.
4. Select the "Contour Size: 0.25" option.
5. Notice that the plot immediately visualizes a much denser set of contours, behaving as it should have upon initialization.

import plotly
import plotly.graph_objects as go

print(f"Plotly version: {plotly.__version__}")

z_data = [
    [10, 10.625, 12.5, 15.625, 20],
    [5.625, 6.25, 8.125, 11.25, 15.625],
    [2.5, 3.125, 5.0, 8.125, 12.5],
    [0.625, 1.25, 3.125, 6.25, 10.625],
    [0, 0.625, 2.5, 5.625, 10],
]

fig = go.Figure(
    data=go.Contour(
        z=z_data,
        contours=dict(size=0.25),
        colorbar=dict(
            title=dict(
                text="Color bar title",
                side="right",
                font=dict(size=14, family="Arial, sans-serif"),
            )
        ),
    )
)

# This assertion passes, proving the property is registered in python-land
assert fig.data[0].contours.size == 0.25

fig.update_layout(
    updatemenus=[
        dict(
            type="dropdown",
            buttons=[
                dict(
                    label="Contour Size: 0.25",
                    method="restyle",
                    args=[{"contours.size": 0.25}],
                ),
                dict(
                    label="Contour Size: 1.0",
                    method="restyle",
                    args=[{"contours.size": 1.0}],
                ),
                dict(
                    label="Contour Size: 2.0",
                    method="restyle",
                    args=[{"contours.size": 2.0}],
                ),
            ],
        )
    ]
)

fig.show()

Notes

• The internal Python object structure correctly holds the contours.size value (as verified by the passing assert fig.data[0].contours.size == 0.25 statement).
• The version of plotly.py is 6.8.0.