Adapt to bokeh 3.8

Noticed when switching desienv from bokeh 2 to 3.8:
- when running prospect I sometimes get (not all the time) a thing I never saw before: `BokehUserWarning: out of range integer may result in loss of precision`,
- some widget layouts aren't set as before in the web page (may depend on browser)

Start by reproducing the reported behavior in prospect after switching from Bokeh 2 to 3.8, checking both the intermittent out-of-range integer warning and the widget layouts in the web page across browsers. Trace each symptom to the relevant prospect entry point, then verify that the warning is gone and layouts match the previous behavior.

Written by the indexing model from the issue text.