Show serial does not immediately show either print() output or error traceback

[martinwork]

Bug Description

Arising from https://support.microbit.org/helpdesk/tickets/87116 (private)

After pasting in code or loading a hex with a print() statement, then sending it to micro:bit, and clicking Show serial, the serial panel remains blank. If there's a code error, the traceback doesn't show either.

Switching to another browser tab and back fixes it.

Example: serial.zip

from microbit import *
count = 0
while True:
    display.show(Image.HEART)
    print( count)
    count += 1
    sleep(1000)
    display.show(Image.HEART_SMALL)
    sleep(1000)

How To Reproduce

Steps to reproduce the behavior:

1. Go to https://python.microbit.org/
2. Paste in the example
3. Send to micro:bit
4. Click Show serial
5. See error

Expected behavior

print() output and traceback appears in the serial window

Environment

Desktop (please complete the following information):

• OS: [e.g. iOS] Windows 10
• Browser [e.g. chrome, safari] Chrome
• Version [e.g. 22] 133

[microbit-matt-hillsdon]

@microbit-grace please can you reproduce this and explore whether this might be a regression due to the connection library update?

[microbit-grace]

The issue seems to have existed pre-introduction of the connection library. I can recreate the issue on v3.0.27 (before introduction of connection library).

Also, the most recent version (v3.1.1) doesn't seem to include out latest connection library update. Though the changes doesn't seem to fix the issue.

Correction
It was in fact due to the connection library changes. See PR that fixes this for more information.

[microbit-grace]

Carlos found that doing the following also seems to fix the serial output:

1. Disconnect using the “Disconnect” option in the “send to micro:bit button”
2. Connect by using the “Connect” option in the “send to micro:bit button”

This narrows down why switching tab might fix the serial output.

Carlos also found the following possibly related issues:

Possibly related issue 1

Steps to repro issue:

1. Go to https://python.microbit.org/
2. Send to micro:bit
3. Click Show serial
4. Type something into serial.

See following in console:

usb.js:442 Uncaught (in promise) fl: Failed to execute 'transferOut' on 'USBDevice': The device must be opened first.
    at j2 (https://python.microbit.org/v/3/assets/index-kbhzV1Jk.js:490:2053)
    at RSe (https://python.microbit.org/v/3/assets/index-kbhzV1Jk.js:490:2660)
    at ASe.withEnrichedErrors (https://python.microbit.org/v/3/assets/index-kbhzV1Jk.js:490:694)

Possibly related issue 2

Steps to repro issue:

1. Go to https://python.microbit.org/
2. Send to micro:bit
3. Click Show serial
4. Refresh page and try to send to micro:bit again.
5. See WebUSB error dialog.

See following in console:

Error: Transfer count mismatch
    at e.<anonymous> (dap.esm.js:15:14314)
    at dap.esm.js:15:1550
    at Object.next (dap.esm.js:15:1654)
    at s (dap.esm.js:15:394)