Serial Monitor displaying null characters for seconds before actual data

[joseportillodev]

OS: MacOS Sonoma 14.4.1
VSCode: 1.88.0 (Universal)
Arduino IDE: 2.3.3-nightly-20240305
Arduino VSCode Extension: v0.6.0
Serial Monitor VSCode Extension: v0.11.0

Bug:
Using the serial monitor with an ESP8266 board (Wemos D1 Mini clone) and a simple test application, it displays a huge amount of null/garbled chars, during several seconds, before displaying the actual data sent by the ESP8266 to the computer.
On many occasions, after the aforementioned huge amount of null/garbled chars, the pane displays some chunk of data (one line or a part of it) repeatedly before actual data.

Notes:

• This happens the first time you connect the serial cable to the board and hit the "Start Monitoring" button.
• This happens right after uploading the sketch (every time).
• This does not happen if you press the reset button on the board (you'll see same garbled characters and some bootloader output indicating reset cause).
• Once this "junk characters phase" finishes, the Serial Monitor pane displays the correct data below it.
• Scrolling up or down the screen through this phase makes scrolling highly slow and stuttering.

When uploading the sketch to the ESP8266, the Arduino extension disconnects the Serial Monitor, performs the sketch upload, and connects the Serial Monitor back. On connection, the pane displays a few more seconds of junk characters and I can finally see the sketch output.

Steps:

• Open Serial Monitor pane on VSCode, select proper port and baudrate. Click "Start Monitoring". Junk scrolls up the screen for a few seconds.
• Upload minimal test application. The Arduino extension automatically disconnects the Serial Monitor, performs the sketch upload, and connects the Serial Monitor back. On connection, the pane displays a few more seconds of junk scrolling up the screen.

Sample sketch:

void setup() {
    // LED ON
    pinMode(LED_BUILTIN, OUTPUT);
    digitalWrite(LED_BUILTIN, LOW);
    delay(1000);

    // Setup serial port
    Serial.begin(115200);
    Serial.println(F("\r\nESP8266 TEST APPLICATION v1.0 | Build " __DATE__ " " __TIME__));
    Serial.println(ESP.getFullVersion());
}

void loop() {
    static int i;
    
    Serial.printf("Loop: %u\r\n", i++);
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    delay(1000);
}

Expected output (sketch banner and loop traces):

What actually happens:

...a few seconds later...

According to the timestamps, only one second has elapsed and only one huge line of <null?> chars has been "inserted" into the pane view.
After a few seconds of wait time, you'll see the application banner and at least 15~20 loop traces appear instantly.

Quick demo video (start Monitoring after connecting board to computer)
https://github.com/microsoft/vscode-arduino/assets/166513883/e988952a-326d-47ea-b21f-103d1b6c00c9

Quick demo video (upload sketch)
https://github.com/microsoft/vscode-arduino/assets/166513883/d5fcb701-e565-4776-8261-8cfa9a9f1a67

Thank you!
Jose

[joseportillodev]

Just one more screenshot, showing the "repeating piece of data" issue.
(just clicked Start Monitoring while the board was connected and the sketch running)