mo.status.progress_bar shows wrong state when some updates are fast

[johannesacco]

Describe the bug

There seem to be some throttling that cause an old state to be shown when some iterations are very fast. Using the internal marimo_output.flush() solves the problem, but I think that should not be needed.

Will you submit a PR?

• Yes

Environment

{
"marimo": "0.24.0",
"editable": false,
"location": "...",
"OS": "Linux",
"OS Version": "7.0.0-30-generic",
"Processor": "x86_64",
"Python Version": "3.13.11",
"Locale": "en_US",
"Binaries": {
"Browser": "152.0.7977.64",
"Node": "--",
"uv": "0.11.7 (x86_64-unknown-linux-gnu)"
},
"Dependencies": {
"click": "8.4.2",
"docutils": "0.23",
"itsdangerous": "2.2.0",
"jedi": "0.19.2",
"markdown": "3.10.3",
"narwhals": "2.24.0",
"packaging": "26.3",
"psutil": "7.2.2",
"pygments": "2.20.0",
"pymdown-extensions": "10.21.3",
"pyyaml": "6.0.3",
"starlette": "1.6.0",
"tomlkit": "0.15.1",
"typing-extensions": "4.16.0",
"uvicorn": "0.52.1",
"websockets": "17.0.1"
},
"Optional Dependencies": {
"loro": "1.13.2",
"pandas": "3.0.5",
"pyarrow": "25.0.1",
"pytest": "9.0.3",
"ruff": "0.15.12"
},
"Experimental Flags": {}
}

Code to reproduce

In below example the progress bar shows "first" and "0/5" for 3 seconds even though it handles "first" and "second" very fast.

items = [(0, "first"),(0, "second"),(3, "third"),(0, "number 4"),(3, "last")]

with mo.status.progress_bar(
    total=len(items),
    title="Dummy test",
    remove_on_exit=True,
) as bar:
    for item in items:
        bar.update(increment=0, subtitle=item[1])
        await asyncio.sleep(item[0])

Adding a call to the internal marimo_output.flush() solves the problem:

import marimo._runtime.output._output as marimo_output

items = [(0, "first"),(0, "second"),(3, "third"),(0, "number 4"),(3, "last")]

with mo.status.progress_bar(
    total=len(items),
    title="Dummy test",
    remove_on_exit=True,
) as bar:
    for item in items:
        bar.update(increment=0, subtitle=item[1])
        marimo_output.flush()
        await asyncio.sleep(item[0])