Link to issue

Closes #5591

Description of change

to_html wraps the figure inside an outer <div> that has no inline style. The inner figure div sets style="height:100%; width:100%;", but because the wrapper has no height, that 100% resolves to zero, and plotly.js then falls back to its hardcoded 450px default.

The result: the documented default of default_height='100%' cannot inherit from a sized parent container. Any figure placed inside a Jinja template, a Dash container with a vh/% height, or any responsive layout silently renders at 450px instead of filling its parent. This is the underlying cause of #5591 (and the long-standing plotly.js#5270). PR #5590 documented a regex-based workaround; this PR addresses the root cause.

The fix moves the requested default_height/default_width onto the outer wrapper, and changes the inner figure div to height:100%; width:100%; so it fills the wrapper:

- <div>
+ <div style="height:{height}; width:{width};">
          {mathjax_script}
          {load_plotlyjs}
              <div id="{id}" class="plotly-graph-div"
- style="height:{height}; width:{width};"></div>
+ style="height:100%; width:100%;"></div>

Behavior matrix

Pixel defaults and standalone-document use cases are byte-for-byte equivalent in observable layout. The only behavioral change is that percentage defaults now actually inherit from a sized parent, which matches the docstring.

Testing strategy

Two regression tests added in tests/test_io/test_html.py:

• test_outer_wrapper_carries_requested_dimensions asserts the wrapper div has the percentage style and the inner figure div is set to 100% of it.
• test_outer_wrapper_respects_explicit_pixel_dimensions asserts pixel defaults flow through unchanged.

Existing test_html_deterministic and the CDN/integrity tests are unaffected because they assert content presence, not the wrapper element.

Notes for review

• This is a small visible structural change to to_html output. If maintainers prefer to gate it behind an opt-in (e.g. responsive_layout=True), I am happy to refactor - just let me know.
• I have not been able to reproduce a downstream consumer (Dash, JupyterLab extension, nbconvert) that depends on the wrapper being unstyled. If you know of one, please flag and I will investigate.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.