Vim doesn't respond to window resize while fzf pane is open

- Category
- [ ] Question
- [x] Bug
- [ ] Suggestion
- OS
- [x] Linux
- [ ] macOS
- [ ] Windows
- [ ] Etc.
- Vim
- [x] Vim
- [ ] Neovim

Steps to reproduce:

1. Open Vim (with minimal fzf config) in a small window.
2. Execute a command such as `:Files` to bring up the fzf pane.
3. Make the window containing Vim bigger.

Observed result: Vim does not resize to fill the available space.

Expected result: Vim resizes to fill the available space.

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I'm sure this could be a Vim bug, but I'm not sure. I couldn't figure out from the source how fzf produces its little pane (by some built-in Vim feature, or otherwise).