[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.

首先使用 Plotly 6.8.0 和 pandas 运行提供的 Python 复现，比较初始的 fixed rangemode 与后续的 relayout 行为。跟踪 range-slider 的初始化和 redraw 路径，以确定为什么灰色条只在初始渲染时出现。完成的标准是 fixed rangemode 初始渲染时不显示这些条，并且切换模式时行为仍然正确。