Timing-related page load issue that leaves UI in broken state (simulator driver init)

[microbit-matt-hillsdon]

Describe the bug
We've spotted what seems to be a load/initialisation timing issue that can leave MakeCode UI in a broken state.

To Reproduce
Steps to reproduce the behavior:

1. Open a new incognito browser
2. Open dev tools and set the network tab data rate to "Slow 4G" (this significantly improves the chance of reproduction)
3. Load https://makecode.microbit.org/#editor (slowly!)
4. Most of the time you should see the UI in the bad state shown below with the following logged error:

main.js:1 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'preload')
    at me.internalLoadHeaderAsync (main.js:1:600490)
    at me.loadHeaderAsync (main.js:1:600276)
    at async me.createProjectAsync (main.js:1:625874)

The error is here in the code.

If you need to try again to reproduce then close all incognito browser windows to start with no cache.

We originally saw this issue in micro:bit classroom in the embedded iframe scenario, where you get the same logged error but MakeCode recovers after showing this message:

In the non-iframe scenario above MakeCode does not recover and the user has to reload the page and hope they avoid the issue next time.

Expected behavior
MakeCode loads as normal.

Screenshots

micro:bit version (please complete the following information):

N/A

Desktop (please complete the following information):

• OS: Mac
• Browser Chrome
• Version 132.0.6834.160 (Official Build) (arm64)

CC @microbit-grace @microbit-robert

[abchatra]

I can't reproduce the problem with slow 4g setting in Edge.

[microbit-matt-hillsdon]

I can't reproduce the problem with slow 4g setting in Edge.

@abchatra please check

• that you definitely have a clear cache/incognito window
• that you open https://makecode.microbit.org/#editor (not the home page)

I just tried on a fresh Edge install on Mac (so with an empty cache) and reproduced it first time with Slow 4G enabled.

It might take a few attempts, be sure to use a new incognito window each time.

[abchatra]

I was trying windows. Will try in Mac and see.

[microbit-matt-hillsdon]

Doubt it's a platform thing but maybe the different hardware will get you different timings so you trip over the issue.

[microbit-matt-hillsdon]

I tried Windows and found it harder to reproduce but it does happen for me reliably if you drop down to the (very slow!) "3G" mode. I suspect it's just about forcing the right sequence of events.