Adding tools to an Overlay via opts has no effect.

import holoviews as hv
hv.extension('bokeh')

overlay = hv.Curve([-1, -2, -3]) * hv.Curve([1, 2, 3])
overlay.opts(tools=['zoom_in'])

To add tools to an Overlay, you need to add them to the elements it contains, one seems to be enough:

(Observed in #6190)