Panics due to wndproc reentrancy

[raphlinus]

You can provoke a panic in the master branch by running the sample example, selecting File Open from the menu, then doing a dynamic monitor change (for example, unplugging an external monitor). This is because Windows requires the wndproc to be reentrant, and our implementation uses a (panicking) RefCell::borrow_mut() for safety.

There's a Reddit thread which goes into some more detail, including pointers to how winit handles this. There's also some discussion on the Zulip.

The upshot is that we want to refactor the way we handle interior mutability. In particular, druid-shell should handle it, so the WinHandler can be called mutably and not need its own interior mutability. There's also followup work to make the file dialog not hold long-lived borrows, but that's significantly lower priority.

This came up because we need a good way to plumb additional resources, particularly a factory for creating text layouts, to user methods.

[raphlinus]

I'm closing this, the panics are fully addressed in the muggle branch, and I'm also satisfied about safety. I'm going to open another issue on finishing the migration from WinHandler to WinCtx.