fix: double event processing in Firefox (#16522)

[laszlokorte]

Firefox seems to garbage collect event objects during event propagation if no global/shared reference to the event object is kept between multiple event handlers. That discards the __root marker set on the event object to early, leading to duplicate processing.

I am not sure what is a good way to test this.

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Playground

pnpm add https://pkg.pr.new/svelte@16527

[Ocean-OS]

Thanks!

[7nik]

Is it me, or the fix doesn't work at all? I guess last_propagated_event is treeshaked because it is never read.

[laszlokorte]

Oh, it worked for me locally but you are right that it seems to be treeshaken

[plgingras88]

fyi - Mozilla acknowledged the severity of the bug in Firefox and prepared a PR to fix it in the current stable release. It was initially classified as a normal (S3) issue to be addressed in the next version, but after realizing it was breaking Svelte and directly impacting our business app with a concrete example, they raised the priority to S2 and work to deliver a fix for the current version.

Meanwhile, thanks to svelte amazing team for being that reactive with some magic work-arounds that made the trick for us !

https://bugzilla.mozilla.org/show_bug.cgi?id=1980081