Title moves off-page when adding new line with `<br>`

Description

When I am plotting normally, sometimes I want to add a new line to the title, and use that as a sub-title. When I do this, it will move the first line of my title up and off the page. This occurs even when I set title_y=1 and title_yref="container" and title_yanchor="top".

Reproducible Example

First, let's define the default plot.

from plotly import express as px
data = px.data.stocks(indexed=True, datetimes=True)
fig = (
    px.line()
    .add_scatter(
        name="Google",
        x=data.index,
        y=data["GOOG"],
        showlegend=True,
    )
    .add_scatter(
        name="Amazon",
        x=data.index,
        y=data["AMZN"],
        showlegend=True,
    )
    .add_scatter(
        name="Netflix",
        x=data.index,
        y=data["NFLX"],
        showlegend=True,
    )
)

When I run this with a title with only 1 line, it looks good:

fig1 = fig.update_layout(
    margin={"t": 40, "b": 10, "l": 10, "r": 10},
    title=dict(
        text="<b>Stock Prices</b>",
        x=0.5,
        xref="container",
        xanchor="center",
        y=0.99,
        yref="container",
        yanchor="top",
    ),
    xaxis_title="Date",
    yaxis_title="Values",
    legend_title="Stock",
)
fig1.show()

However, when I change only the title to add a second line, it will push the top of the title off the page.

fig2 = fig1.update_layout(title_text="<b>Stock Prices</b><br>For years 2018-2019")
fig2.show()

It will even have the same problem when I leave the margins as their default values.

fig3 = fig2.update_layout(margin=None)
fig3.show()

If I manually move the title downwards, it will resolve the issue. However, that is not the point here. Because the title should already be anchored to the top of the container at y level 0.99. I shouldn't need to manually move it downwards even more when I add a new line to the title.

fig4 = fig2.update_layout(
    margin={"t": 40, "b": 10, "l": 10, "r": 10},
    title_y=0.9,
)
fig4.show()

Conclusion

Something whacky is happening to the title whenever we set it to run with two lines. Even if we lock it to the top and with the reference to the container, it will still somehow be overwritten.

Please help to fix it.

Additional information

I include the schema files for each plot:

plot1_schema.json.txt
plot2_schema.json.txt
plot3_schema.json.txt
plot4_schema.json.txt

And the full script for entirely reproducible example:

# Import
from plotly import express as px
import json

# Data
data = px.data.stocks(indexed=True, datetimes=True)

# Default figure
fig = (
    px.line()
    .add_scatter(
        name="Google",
        x=data.index,
        y=data["GOOG"],
        showlegend=True,
    )
    .add_scatter(
        name="Amazon",
        x=data.index,
        y=data["AMZN"],
        showlegend=True,
    )
    .add_scatter(
        name="Netflix",
        x=data.index,
        y=data["NFLX"],
        showlegend=True,
    )
)

# Figure 1: Works well
fig1 = fig.update_layout(
    margin={"t": 40, "b": 10, "l": 10, "r": 10},
    title=dict(
        text="<b>Stock Prices</b>",
        x=0.5,
        xref="container",
        xanchor="center",
        y=0.99,
        yref="container",
        yanchor="top",
    ),
    xaxis_title="Date",
    yaxis_title="Values",
    legend_title="Stock",
)
fig1.show()
fig1.write_image("plot1.png", engine="kaleido")
with open("plot1_schema.json.txt", "wt") as f:
    f.write(json.dumps(fig1.layout.to_plotly_json(), indent=2))

# Figure 2: Title off page
fig2 = fig1.update_layout(title_text="<b>Stock Prices</b><br>For years 2018-2019")
fig2.show()
fig2.write_image("plot2.png", engine="kaleido")
with open("plot2_schema.json.txt", "wt") as f:
    f.write(json.dumps(fig2.layout.to_plotly_json(), indent=2))

# Figure 3: Even works with margins are default
fig3 = fig2.update_layout(margin=None)
fig3.show()
fig3.write_image("plot3.png", engine="kaleido")
with open("plot3_schema.json.txt", "wt") as f:
    f.write(json.dumps(fig3.layout.to_plotly_json(), indent=2))

# Figure 4: Works if you manually move the title down; but that's not the point. It should be anchored to the top of the `container` already.
fig4 = fig2.update_layout(
    margin={"t": 40, "b": 10, "l": 10, "r": 10},
    title_y=0.9,
)
fig4.show()
fig4.write_image("plot4.png", engine="kaleido")
with open("plot4_schema.json.txt", "wt") as f:
    f.write(json.dumps(fig4.layout.to_plotly_json(), indent=2))

从 issue 中完整且可复现的 Python 脚本开始，尤其关注 fig1–fig4 的 update_layout 调用以及随附的 plot1_schema.json.txt 至 plot4_schema.json.txt 文件。比较单行标题和多行标题的行为与 schema；当两行标题锚定在容器顶部时仍能完全显示，即视为完成。