Fix undesired seek in overlapping time ranges

[defagos]

Description

This PR fixes #861.

Changes made

• Add test reproducing the issue.
• Fix the issue by applying the same flattening approach as for interstitials. To avoid repeatedly performing the same work, flattening is made once and the result cached.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).