link_selections does not work on complex Dynamic Map

Is your feature request related to a problem? Please describe.

hv.selection.link_selections does not work on complex DynamicMaps. For example, the following code:

from collections import OrderedDict
import numpy as np
import holoviews as hv
import panel as pn

hv.extension('bokeh')


# Widgets
x_offset_slider = pn.widgets.FloatSlider(
    name='x-offset', 
    start=-5, end=5, step=0.1, value=0.
)
y_offset_slider = pn.widgets.FloatSlider(
    name='y-offset', 
    start=-5, end=5, step=0.1, value=0.
)
widget_box = pn.WidgetBox(
    x_offset_slider,
    y_offset_slider
)


@pn.depends(
    x_offset=x_offset_slider,
    y_offset=y_offset_slider,
)
def gen_points_overlay(x_offset, y_offset):
    points_dct = OrderedDict([
        ('p1', hv.Points(np.random.randn(1000, 2), kdims=['x', 'y'], label='p1')),
        ('p2', hv.Points(np.random.randn(1000, 2) + np.array([x_offset, y_offset]), kdims=['x', 'y'], label='p2'))
    ])
    return hv.NdOverlay(points_dct, kdims=['cat'])


dmap_2d = hv.DynamicMap(
    gen_points_overlay, kdims=['x_offset', 'y_offset']
).opts(
    hv.opts.Points(framewise=True)
)


dmap_hist = dmap_2d.map(map_fn=lambda p: hv.operation.histogram(p, dimension='x'), specs=hv.Points)

linked_plot = hv.selection.link_selections(dmap_2d + dmap_hist)

app = pn.Column(linked_plot, widget_box)
app

Results in: Notice the absence of the selection tools.

After some debugging I think the underlying issue is that link_selections creates a new DynamicMap. However, somehow the original and selection DynamicMaps are not correctly combined, and I think it’s because this (which should probably raise a warning?): https://github.com/holoviz/holoviews/blob/effaf45f9c2e7bf33c9f7ee830c2aa355badaae2/holoviews/selection.py#L180-L182

If I understand everything correctly, it seems like it is currently impossible to combine link_selections with a DynamicMap from a custom function. However, I’m wondering if there is a workaround by embedding the selection directly into the custom function? Would anyone be able to help me find a workaround?

Describe the solution you’d like

Support for hv.selection.link_selections on complex DynamicMaps.

Describe alternatives you’ve considered

Extending DynamicMap.map() to add new streams so I can manually add a new selection stream on top of the existing DynamicMap.

[peterroelants]

warning added in https://github.com/holoviz/holoviews/pull/5044 to make clear that link_selections does not work on complex Dynamic Map.

[peterroelants]

I’m willing to have a look to see if I can cover more cases, and the above case more specifically, if anyone has some suggestions on how to cover these.