Layout shenanigans may crash zellij

[raphCode]

Like already reported in #1289, doing some evil stuff with the layout may lead to some trouble:

1. create a 3x3 layout with the splits aligned (unequal pane sizes are fine)
2. resize the panes directly touching the center pane (von Neumann neighborhood 🤓). This "locks up" some of the split lines:

• top one left
• left one down
• bottom one right
• right one up

3. close the panes in the 4 corners
4. close the center pane

Screenshots of the layout these steps should create

After resizing (2.):

After closing the corner panes (3.):

Zellij is then in an unstable state:

1. The UI still shows the closed center pane
2. Trying to type something without changing the pane focus crashes zellij
3. Switching between different tabs freezes content where the center pane used to be
4. resizing the terminal windows flickers the pane tiling: it switches rapidly between two splits

The panic of 2.:

Error occurred in server:

  × Thread 'screen' panicked.
  ├─▶ Originating Thread(s)
  │   	1. stdin_handler_thread: AcceptInput
  │   	2. screen_thread: WriteCharacter
  │
  ├─▶ At zellij-server/src/tab/mod.rs:835:60
  ╰─▶ called `Option::unwrap()` on a `None` value

[imsnif]

What would the right thing to do here be? Open a new pane in its place?

[raphCode]

Good question, but keeping users from closing panes might be a bad experience.

In observation 4, the layout engine even has found a solution:

The problem is that the continuous vertical center split flickers around when resizing the terminal window. Making the layout engine decide on a single split position would be okay for me.

Alternatives solutions which are fine:

• the analogous layout with the common split line running horizontally
• a completely symmetric solution is a 2x2 layout with aligned splits.
  This might be the most logical, because you arrive at that solution when shrinking the center pane to zero size. The surrounding panes grow towards each other like a closing aperture:
  
  Also this leaves users with the most freedom because they can continue resizing in any direction.