KBFS breaks thunar and apt upgrade on Debian

While /keybase is mounted (the default after installing the keybase client) `apt upgrade` stalls at
```
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
```

Also thunar (the XFCE4 file manager) never opens a window.

Doing
```
sudo umount -f /keybase
```
fixes both these issues.

When running I see
```
$ mount | grep keybase
keybase-redirector on /keybase type fuse (ro,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other)
/etc/auto.net on /net type autofs (rw,relatime,fd=7,pgrp=73335,timeout=300,minproto=5,maxproto=5,indirect,pipe_ino=329190)
/dev/fuse on /run/user/1003/keybase/kbfs type fuse (rw,nosuid,nodev,relatime,user_id=1003,group_id=50)
```

I'm running Debian Unstable on x86_64, using the XFCE4 desktop environment.

Reproduce the issue on Debian Unstable with /keybase mounted, then run apt upgrade and open Thunar before and after `sudo umount -f /keybase`. Compare the behavior and identify what needs to change so both operations work while the KBFS mount remains active.

Written by the indexing model from the issue text.