Sometimes not rebuilding after update

Since some version already not building after source update.
This used to work till before two - three week without problem and now
sometimes I need to stop the webdev serve and rerun after
making changes to source which is not very comfortable.

Start by reproducing the problem with `webdev serve`, editing source files until a change fails to rebuild. Trace the source-watching and rebuild entry points, then verify that source updates consistently rebuild without stopping and restarting the server.

Written by the indexing model from the issue text.