DatetimeSlider / DateSlider / DatetimeRangeSlider render an empty container (no slider track) when served — no console errors #8742

Description

DatetimeSlider, DateSlider and DatetimeRangeSlider render an empty container when served via pn.serve (Bokeh server): the widget's DOM node exists and the slider title/label renders, but the slider track itself never appears — there is nothing to drag. IntSlider / DiscreteSlider render fine on the same page.

No JS console errors are logged; BokehJS reports Bokeh items were rendered successfully.

Versions tested (all reproduce)

(panel 1.9.4 requires bokeh>=3.7.0,<3.10.0, so bokeh 3.10 is unreachable with current panel.)

Minimal repro

from datetime import datetime

import panel as pn

pn.extension()
slider = pn.widgets.DatetimeSlider(
    name="t",
    start=datetime(2026, 8, 21, 8, 45),
    end=datetime(2026, 8, 21, 13, 30),
    value=datetime(2026, 8, 21, 8, 45),
)
pn.serve(slider, port=5203, show=True)

Verification (browser devtools)

// shadow-DOM piercing query:
const collect = (root, out) => {
  for (const e of root.querySelectorAll("*")) { out.push(e); if (e.shadowRoot) collect(e.shadowRoot, out); }
  return out;
};
const dt = collect(document, []).find(e => typeof e.className === "string" && e.className.includes("-DatetimeSlider"));
dt.children.length   // → 0
dt.outerHTML.length  // → 66 (just the empty <div class="bk-panel-models-datetime_slider-DatetimeSlider"></div>)

The slider title element (bk-slider-title) does render, which makes the widget look present — but the noUi track/handles are never attached, so it cannot be interacted with.

Expected behavior

An interactive date slider, as IntSlider/DiscreteSlider behave on the same page.

Workaround

DiscreteSlider with string options (e.g. HH:MM labels mapped to timestamps) works correctly.