In case somebody is interested, I found the problem: I was indeed
using the wrong event handler!
I was collecting data for added and changed source files in
"env-get-outdated" and stored it as attribute of the `env` variable.
However, I was also using "env-purge-doc", which was running after
"env-get-outdated" and was deleting all my data again!
That's why it wasn't available in `app.env` in "html-page-context".
To solve this I have moved my data collection from "env-get-outdated"
to "env-updated". In order to know which source files have changed,
I'm using "source-read" to mark new/changed files.
Finally, to make this also work for parallel builds, I use
"env-merge-info" to merge all information obtained in "source-read".
Now it works as expected!