Can't install modules from UI when site placed in subdirectory

Question

I have Drupal 8 installed in a subfolder (www.example.com/drupal), and I have my root .htaccess configured to mask the subfolder (i.e. the URLs resolve to www.example.com; /drupal does not appear in the URLs).

Everything works great except when I try to install new modules I get a 403 error. I see in the URL for authorize.php that /drupal now appears in the URL. Where I would expect www.example.com/core/authorize.php/, I instead see www.example.com/drupal/core/authorize.php/. I imagine this is the reason for the 403?

What additional modifications do I have to make to /drupal/.htaccess in order to get access to the /core files required to install new modules?

Here's what's in my document root .htaccess:

RewriteEngine On
RewriteCond %{REQUEST_URI} !drupal/
RewriteRule (.*) /drupal/$1 [L]

And the only modification to /drupal/.htaccess is to uncomment the subdirectory per the instructions in the file:

# Modify the RewriteBase if you are using Drupal in a subdirectory or in a
# VirtualDocumentRoot and the rewrite rules are not working properly.
# For example if your site is at http://example.com/drupal uncomment and
# modify the following line:
RewriteBase /drupal

Answer 1

You have to uncomment this line.

  # Modify the RewriteBase if you are using Drupal in a subdirectory or in a
  # VirtualDocumentRoot and the rewrite rules are not working properly.
  # For example if your site is at http://example.com/drupal uncomment and
  # modify the following line:
  # RewriteBase /drupal

Another option is to update module using drush.

drush up