Regression in hstack flexbox behaviour

[ms-pablo-ruiz]

Describe the bug (Updated version after narrowing down the cause)

Seems that the update #8219 have changed the default behavior of boxes inside of stacks:

mo.hstack(
    [mo.stat(value=1, bordered=True), mo.stat(value=2, bordered=True), mo.stat(value=3, bordered=True)],
    #justify="space-between",
    widths="equal",
)

It used to be that providing widths="equal", would be equivalent to use flex-grow:1 in a flexbox, the elements inside would grow horizontally and take all the space in between, but not anymore. You can see it even the example in the docs, is basically broken:

https://docs.marimo.io/api/layouts/stacks/?h=vstack

Now there is no easy way of making the boxes to stretch taking all the horizontal space. The items inside the flexbox are made containers itself, which is not correct.

Will you submit a PR?

• Yes

Environment

Details

{
  "marimo": "0.19.11",
  "editable": false,
  "location": "/Users/pablo.ruiz/Documents/code/jd_autotrading_app/.venv/lib/python3.13/site-packages/marimo",
  "OS": "Darwin",
  "OS Version": "24.6.0",
  "Processor": "arm",
  "Python Version": "3.13.1",
  "Locale": "C/en_US",
  "Binaries": {
    "Browser": "145.0.7632.46",
    "Node": "v22.16.0",
    "uv": "0.9.0 (39b688653 2025-10-07)"
  },
  "Dependencies": {
    "click": "8.3.1",
    "docutils": "0.22.4",
    "itsdangerous": "2.2.0",
    "jedi": "0.19.2",
    "markdown": "3.10.2",
    "narwhals": "2.16.0",
    "packaging": "26.0",
    "psutil": "7.2.2",
    "pygments": "2.19.2",
    "pymdown-extensions": "10.21",
    "pyyaml": "6.0.3",
    "starlette": "0.52.1",
    "tomlkit": "0.14.0",
    "typing-extensions": "4.15.0",
    "uvicorn": "0.40.0",
    "websockets": "16.0"
  },
  "Optional Dependencies": {
    "altair": "6.0.0",
    "duckdb": "1.4.4",
    "loro": "1.10.3",
    "openai": "2.21.0",
    "pandas": "3.0.0",
    "polars": "1.38.1",
    "pyarrow": "23.0.0",
    "pytest": "8.4.2",
    "python-lsp-ruff": "2.3.0",
    "python-lsp-server": "1.14.0",
    "ruff": "0.15.1",
    "sqlglot": "28.10.1"
  },
  "Experimental Flags": {}
}

Code to reproduce

mo.hstack(
    [mo.stat(value=1, bordered=True), mo.stat(value=2, bordered=True), mo.stat(value=3, bordered=True)],
    #justify="space-between",
    widths="equal",
)

[ms-pablo-ruiz]

Just to narrow down, last confirmed version working is 0.19.9, so the bug is introduced in the 0.19.10, also I think it breaks the widths functionality completely

[ms-pablo-ruiz]

Main candidate of the reggresion:

#8219

[ms-pablo-ruiz]

Confirmed #8219 introduced the reggresion

[ms-pablo-ruiz]

The regression is because now we are making every item inside a flex box a container (when width is provided), which is not correct. It does work in nested vstacks but not in non nested hstacks for instance.