Add to Panel menu: “Minimize this Panel”. Because there seems to be no way to do this explicitly; it just happens as a side-effect of switching to different panels.
Sometimes when I have a few visible, I want to work with e.g. two specific panels, but if I open one, the other one gets minimized 
The one thing that keeps panels from being totally awesome is a lack of control.
I need to be able to decide WHICH two panels will be displayed full-size, but right now it is basically a side-effect of other actions. So in addition to being able to Minimize a panel, I’d like to “Pin” a panel to make it stay open (and only one can be pinned at a time).
The most intuitive behavior IMO would be that the LAST TWO panels I clicked would be be visible (unless there is a pinned panel). Right now, two panels must be next to each other in order to both be visible.
Also I would like to be able to open a View from the Left menu without losing all my panels - i.e. have Views open in panels, too.
Agreed. I begin to wonder if trying to do any automated behavior is really worth the iteration on it, or if instead it should be made highly accessible in an intuitive set of UI actions and keyboard shortcuts, like Obsidian and a number of other tools that support panel-like (e.g. sidebar) functionality.
I totally love the idea of being able to Minimize and Pin panels. One of the main issues in Fibery in my experience is loss of context. See also this related other topic about multi-directional panel navigation.
Likewise, pinned and minimized panels will help for:
Context Preservation: When working on complex projects, keeping multiple panels open can help in maintaining the context and reducing the cognitive load of having to remember details from other parts of the project.
Quick Access: Pinned panels can serve as quick shortcuts to frequently used functionalities, reducing the time spent in navigation.
Multi-Tasking : Being able to minimize or pin panels would allow users to switch between tasks more efficiently without losing context.
Other idea
I could also imagine that ‘Panel collections’ would be useful, since I often need to work on a specific set of entities for a day or some days, which would allow me to quickly boot up these panels in the right order.