[BUG]: Range slider y-axis "fixed" rangemode renders unexpected thick horizontal bars on initial load

Description

When initializing a Plotly chart with an xaxis range slider that has its yaxis.rangemode set to "fixed", the chart initially renders two extra, thick, solid gray horizontal bars stretching across the entire width of the range slider track.

However, this appears to be a visual artifact or initialization bug: if you dynamically toggle the rangemode to "auto" or "match" (via an updatemenu dropdown) and then switch it back to "fixed", the gray bars disappear entirely, and the range slider track displays normally as expected.

Screenshots/Video

Initial render (Buggy):

After toggling to "Auto"/"Match" and back to "Fixed" (Expected):

Steps to reproduce

1. Run the provided Python script to generate the Plotly figure.
2. Observe the range slider at the bottom of the chart upon initial rendering. Note the unexpected thick gray horizontal bars spanning the slider track.
3. Use the dropdown menu in the top-left corner to select "Rangemode: Auto/Match".
4. Use the dropdown menu again to select "Rangemode: Fixed".
5. Observe that the range slider now looks correct and the gray bars have vanished.

import plotly; print(plotly.__version__) # 6.8.0
import pandas as pd

pd.options.plotting.backend = "plotly"

df = pd.DataFrame(dict(a=[1, 3, 2], b=[3, 2, 1]))
fig = df.plot.line()

fig.update_layout(xaxis=dict(rangeslider=dict(visible=True, yaxis=dict(rangemode="fixed"))))

fig.update_layout(
    updatemenus=[
        dict(
            type="dropdown",
            active=1,
            buttons=[
                dict(
                    label="Rangemode: Auto",
                    method="relayout",
                    args=[{"xaxis.rangeslider.yaxis.rangemode": "auto"}],
                ),
                dict(
                    label="Rangemode: Fixed",
                    method="relayout",
                    args=[{"xaxis.rangeslider.yaxis.rangemode": "fixed"}],
                ),
                dict(
                    label="Rangemode: match",
                    method="relayout",
                    args=[{"xaxis.rangeslider.yaxis.rangemode": "match"}],
                ),
            ],
        )
    ]
)

fig.show()

Notes

• Environment: Verified using plotly version 6.8.0 alongside pandas plotting backend.
• This behavior suggests a missing or improper initialization redraw when rangemode="fixed" is passed via the initial layout configuration, which is subsequently corrected whenever relayout triggers a fresh calculation of the range slider boundaries.

Inizia eseguendo la riproduzione Python fornita con Plotly 6.8.0 e pandas, confrontando il fixed rangemode iniziale con il comportamento successivo di relayout. Segui l’inizializzazione del range-slider e il percorso di redraw per identificare perché le barre grigie compaiono solo nel rendering iniziale. Il lavoro è completato quando il fixed rangemode viene renderizzato inizialmente senza le barre e il comportamento rimane corretto passando da una modalità all’altra.