Modifying a child element in a tab causes the whole tab to rerender

ALL software version info (bokeh, python, notebook, OS, browser, any other relevant packages)

Bokeh 1.0.1

Description of expected behavior and the observed behavior

I’m not sure if this is expected, but modifying the children of say a column, or really any layout container, where said column/container is within a tab, causes the whole Tabs element to be invalidated/rerendered, instead of just that container. This seems like a pretty poor design for how tabs works, because this is really slow if the tabs contain lots of plots (compare to just having columns and rows - updating a one does not invalidate siblings)

Complete, minimal, self-contained example code that reproduces the issue

from bokeh.models.widgets import Panel, Tabs, Button
from bokeh.layouts import row
from bokeh.plotting import figure

p1 = figure(plot_width=300, plot_height=300)
p1.circle([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], size=20, color="navy", alpha=0.5)
b = Button(label='click')
r = column(column(b, p1))
tab1 = Panel(child=r, title="circle")

p2 = figure(plot_width=300, plot_height=300)
p2.line([1, 2, 3, 4, 5], [6, 7, 2, 4, 5], line_width=3, color="navy", alpha=0.5)
tab2 = Panel(child=p2, title="line")

tabs = Tabs(tabs=[ tab1, tab2 ])

#modifying children causes `tabs` to be completely rerendered
b.on_click(lambda: r.children.append(Button(label='new button')))
layout = tabs

curdoc().add_root(layout)

[mattpap]

This may be already fixed in PR #8085.

[mattpap]

Also make sure to tab headers’ scroll doesn’t jump around after modifying tabs, especially after closing tabs (PR #8732).