Cache seems invalidated because of a suspicious date

[MangelMaxime]

Discussed in #3978

^{Originally posted by joprice December 17, 2024}
I'm curious whether the current behavior I'm seeing is correct when rebuilding after a single line change. I find that when I run fable with a command like

dotnet fable cli/Cli.fsproj -e .fs.js -o cli/js --runScript "notifications"

the first build takes about 15s. The second is less than a second and finishes with Skipped compilation because all generated files are up-to-date!,

but after a single line change, I see output that seems to indicate the entire project is being recompiled (Compiled 221/224, etc), including files in the fable_modules folder, which, of course, I didn't modify. The time taken is the same as the full recompile, which makes me think that build caching only works for the exact same input, and not on incremental changes, or my environment or build is somehow misconfigured causing this.

Is this to be expected and if so, is there any way to improve it? It's my main frustration right now working with fable, so I'd be willing to contribute some time if it could help.