Scattergl and px.line fill and continuous error bands don't appear

Summary

Using the fill and fillcolor parameters on scattergl traces (to produce continuous error bands, for example) does not work.

Issue

The issue is specific to scattergl traces: the same parameters work fine with scatter traces. To avoid the problem with plotly.express px.line() calls requires using render_mode="svg" to avoid the use of scattergl traces.

Using graph_objects has the some problem: one must use go.Scatter() and not go.Scattergl() to get error bands.

The reference for scattergl traces (https://plotly.com/python/reference/scattergl/#scattergl-fill) indicates that the behaviour should be the same as for scatter.

Specs

Python Version: 3.8.6
Plotly Version: 4.14.3
Kaleido Version: 0.2.1

• For image exports to be pasted here (they all looked the same as rendered in the browser)

Browsers: Firefox 88.0, Safari 12.1.1

Example

import numpy as np
import pandas as pd
import plotly.express as px

# dummy data
x = np.linspace(0, 10000, num=10_000)

df = pd.DataFrame({'x': x, 'y': x**2})
df['y_err_plus'] = df['y'] * 1.1
df['y_err_minus'] = df['y'] * 0.95

# use webgl and no fill

fig = (
px
 .line(
    df, x='x', y=['y', 'y_err_minus','y_err_plus'], 
    render_mode='webgl')
 .update_traces(
     fill='tonexty', fillcolor='red',
     selector={'name':'y_err_plus'}
 )
)
fig

# use svg to get fill

fig = (
px
 .line(
    df, x='x', y=['y', 'y_err_minus','y_err_plus'], 
    render_mode='svg')
 .update_traces(
     fill='tonexty', fillcolor='red',
     selector={'name':'y_err_plus'}
 )
)
fig

Sanity Check: fill to zero works?

Yes.

fig = (
px
.line(
    df, x='x', y='y', 
    render_mode='webgl')
.update_traces(fill='tozeroy')
)

fig

Reproduce the issue with px.line(render_mode="webgl") or go.Scattergl() using fill="tonexty" and fillcolor, then compare it with the working scatter and render_mode="svg" cases. Trace the scattergl rendering path and existing fill handling; done means continuous error bands render in webgl while the existing tozeroy behavior remains correct.