Update: actually I think this is broken in Chrome as well...

confirmed in Linux Mint with FF 138.0.1

The scrolling threshold, below which the back-button is broken, seems to be the moment the content loaded in the astro-island tag appears on the page.

This is the console log when pressing the back-button.

Source map error: Error: URL constructor: is not a valid URL.
Stack in the worker:resolveSourceMapURL@resource://devtools/client/shared/source-map-loader/utils/fetchSourceMap.js:56:22
getOriginalURLs@resource://devtools/client/shared/source-map-loader/source-map.js:73:24
workerHandler/</<@resource://devtools/client/shared/worker-utils.js:115:52
workerHandler/<@resource://devtools/client/shared/worker-utils.js:113:13

Resource URL: about:srcdoc
Source Map URL:

Maybe related to withastro/astro#11919 ?

Thanks for the investigation, both! @antiboredom if you have time after 15th and if no one else volunteers, please ping here, happy to assign.

I did a git bisect as I know this wasn't the behavior before, I tracked it down to be introduced by #812. I need to look up what preact is doing in this case but it seems to be pushing history entry whenever a sketch is unloaded on the page.

Ok I think I found the root cause although it is not definitive. Every navigation within an iframe will be counted as a history entry in the top most document, typically changing the src attribute counts as a navigation as well which leads to a behavior in React/Preact where dynamic src attribute will cause iframe to navigate and add entries to the history API.

However we are not using the src attribute rather the srcdoc attribute that directly sets the HTML content of the iframe. In the current version when we unload the content of the iframe, we are setting srcdoc to undefined, that is interpreted by preact as a navigation because in practice it removes the srcdoc attribute entirely, and in my guess/understanding, is understood as a change in src and thus a navigation. What we can do instead is to set srcdoc to "" (blank string) instead which will preserve the srcdoc attribute while still unloading the content of the iframe, from my test, this is not counted as a navigation and does not add an entry to the history API.

@davepagurek Seeing as you did the original implementation, can you have a look at the above to see if it made sense to you. I'll make a PR to fix as such.