Be able to collapse and expand Pools / Participants without loosing contents

Users often request a pool / participant to be collapsable, similar to sub-processes.

Use Case

Under certain circumstances a user would like to reduce the complexity of the shown collaboration diagram by collapsing individual pools. Each pool should keep its content so that it can be expanded again later on. Message flows in and out of the pool should remain visible, too, docking to the pool borders rather than to hidden elements.

Full spec: https://github.com/bpmn-io/bpmn-js/issues/736.

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Referenced via SUPPORT-5274

[volkergersabeck]

One issue with our current implementation of collapsing pools is that people do not realize that this also deletes all its content. They might realize this only when they expand the pool again and find it being empty.

A quick fix for this could be to disallow collapsing of pools if they are not empty. This way, we could prevent the negative surprise of large diagram content being deleted.

[pinussilvestrus]

I think we should keep this issue for its original purpose, namely collapsing and expanding Pools without losing contents, which we are not going to build any time soon. Let’s move it to backlog then.

Rationale: saving the contents is still a valid feature request, but not related to the approach we will go for as a next step. We could consider to close this one and keep the feature request as a general bpmn.io concern (https://github.com/bpmn-io/bpmn-js/issues/736).

I created another issue to track the work regarding the last discussions: https://github.com/camunda/camunda-modeler/issues/2022