Recalculating outputs don't fade

[jcheng5]

After the initial load of the output, move the slider.

Expected: The output should be semitransparent as we recalculate
Actual: The output stays opaque

from shiny import *
import asyncio

app_ui = ui.page_fluid(
    ui.input_slider("n", "N", 0, 100, 40),
    ui.output_text_verbatim("txt"),
)

def server(input, output, session):
    @output()
    @render_text()
    async def txt():
        await asyncio.sleep(5)
        return f"The value of n*2 is {input.n() * 2}"

app = App(app_ui, server)

[wch]

I think this is because we're not sending busy messages.

Here's the log from an R Shiny app:

SEND {"config":{"workerId":"","sessionId":"ea92ac937318c12ce640fcd4834af408","user":null}}
RECV {"method":"init","data":{"bins":30,".clientdata_output_distPlot_width":857,".clientdata_output_distPlot_height":400,".clientdata_output_distPlot_bg":"rgb(255, 255, 255)",".clientdata_output_distPlot_fg":"rgb(51, 51, 51)",".clientdata_output_distPlot_accent":"rgb(51, 122, 183)",".clientdata_output_distPlot_font":{"families":["Helvetica Neue","Helvetica","Arial","sans-serif"],"size":"14px"},".clientdata_output_distPlot_hidden":false,".clientdata_pixelratio":2,".clientdata_url_protocol":"http:",".clientdata_url_hostname":"127.0.0.1",".clientdata_url_port":"3781",".clientdata_url_pathname":"/",".clientdata_url_search":"",".clientdata_url_hash_initial":"",".clientdata_url_hash":"",".clientdata_singletons":""}}
SEND {"busy":"busy"}
SEND {"recalculating":{"name":"distPlot","status":"recalculating"}}
SEND {"recalculating":{"name":"distPlot","status":"recalculated"}}
SEND {"busy":"idle"}
SEND {"errors":{},"values":{"distPlot":{"src":"data:image/png;[base64 data]","width":857,"height":400,"alt":"Plot object","coordmap":{"panels":[{"domain":{"left":40.88,"right":98.12,"bottom":-1.08,"top":28.08},"range":{"left":118.08,"right":1653.52,"bottom":652.12,"top":117.08},"log":{"x":null,"y":null},"mapping":{}}],"dims":{"width":1714,"height":800}}}},"inputMessages":[]}

And one from a PyShiny app:

SEND: {"config": {"workerId": "", "sessionId": "1", "user": null}}
RECV: {"method":"init","data":{"n":40,".clientdata_output_txt_hidden":false,".clientdata_pixelratio":2
,".clientdata_url_protocol":"https:",".clientdata_url_hostname":"connect.rstudioservices.com",".client
data_url_port":"",".clientdata_url_pathname":"/content/5eb87cc3-f23d-4656-96ce-5b0a9359a50c/edit/app_2
fn637hff0gmee183egq/",".clientdata_url_search":"",".clientdata_url_hash_initial":"",".clientdata_url_h
ash":"",".clientdata_singletons":""}}
SEND: {"recalculating": {"name": "txt", "status": "recalculating"}}
SEND: {"recalculating": {"name": "txt", "status": "recalculated"}}
SEND: {"values": {"txt": "The value of n*2 is 80"}, "inputMessages": [], "errors": {}}

Also see #16.