slow callbacks are all pruned in the presence of a dcc.Interval

If the callbacks triggered by a dcc.Interval take longer to complete than the refresh time, they all get pruned and nothing updates.

Example app:

import dash
from dash.dependencies import Input, Output
import dash_core_components as dcc
import dash_html_components as html
import time
app = dash.Dash(__name__)
app.layout = html.Div([
    dcc.Interval(id='i', interval=2000),
    html.Div(id='o')
])
i = 0
@app.callback(Output('o', 'children'), Input('i', 'n_intervals'))
def update_output(n):
    global i
    i += 1
    print([n, i])
    time.sleep(5)
    print([n, i])
    return str([n, i])
app.run_server(debug=True)

dash                 1.20.0
dash-core-components 1.16.0
dash-html-components 1.1.3
dash-renderer        1.9.1
dash-table           4.11.3

Describe the bug

The page never updates: callbacks are all pruned.

From the network tab:

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Expected behavior

One callback should eventually complete.

Reproduce the example app with dcc.Interval, the callback, and the network-tab request pattern shown in the issue. Trace how overlapping interval-triggered callbacks are pruned, then verify that at least one callback completes and updates the output when its runtime exceeds the interval.