What
Installing a plugin archive now extracts it to a temporary staging directory first, then swaps it into place atomically, keeping a backup of the previous version during the swap. On Windows, if files are locked by another process (errors like EPERM, EACCES, or EBUSY), it retries, and if the directory is still locked afterward, it falls back to extracting in place with a warning rather than failing the install.
Why
This makes plugin installs more resilient, especially on Windows where antivirus software or other processes commonly hold files open, and reduces the chance of a failed or partially-applied install.