configure .htaccess to use a subfolder as root and mask the url - .htaccess

Question

I've been trying several solutions in the internet and can't make the redirect voodoo to work.
Setup
My setup is a website build with jekyll, and I want to consume it as mydomain.com/~user.
The folder structure is
~user
- _site
- index.html
- folder1
- index.html
- folder2
- index.html
- other_stuff
Problem
I was able to redirect from mydomain.com/~user to mydomain.com/~user/_site. So, half way through. The problem came when I tried to mask and remove the _site from the URL again. And I hit a wall.
Basically I want to use: mydomain.com/~user and see that URL in the browser, while beeing served the page that lives in mydomain.com/~user/_site/index.html. And same goes for other links
Request See Served
======= ======= =======
mydomain.com/~user mydomain.com/~user mydomain.com/~user/_site/
mydomain.com/~user/folder1 mydomain.com/~user/folder1 mydomain.com/~user/_site/folder1
What I try
RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/~user/_site/?
RewriteRule ^(.*)$ /~user/_site/$1
RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain\.com$
RewriteRule !^~user/_site(.*)$ /~user/_site$1 [L]
RewriteEngine On
RewriteCond %{HTTP_HOST} ^mydomain\.com$
RewriteRule ^~user/_site(.*)$ /~user$1 [L,R=301]
RewriteCond %{HTTP_HOST} ^mydomain\.com$
RewriteRule !^~user/_site(.*)$ /~user/_site$1 [L]
However, all these
Do not end with the desired hidden URL in the bar
Give an error, as some of the redirects are not allowed in the server
Partial solution
After tweaking the proposed solution, I was able to identify one variation that partially works
RewriteEngine On
RewriteBase /~user
RewriteCond %{HTTP_HOST} ^(www\.)?mydomain\.com$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/_site/? [NC]
RewriteRule ^(.*)$ _site/$1 [L]
Note that in the last two line, i remove the ~user from the regex. This redirects all subfolders and pages, and allows other folders that are in ~user to be accessed.
However, the "home" mydomain.com/~user is the only one that is not redirected. As it exists the RewriteCond %{REQUEST_FILENAME} !-d doesn't allow it to be processed.
RewriteCond %{HTTP_HOST} ^(www\.)?mydomain\.com$ [NC]
RewriteCond %{REQUEST_URI} ^/$ [NC]
RewriteRule ^/?$ _site/ [L]
To redirect the home (~user) to mydomain/~user/_site, but it doesn't work. And tried variations of ^/?$ and ^$. But they don't work either.
Ideas?

Answer 1

You should try it this way.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?mydomain\.com$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/~user/_site/? [NC]
RewriteRule ^~user/(.*)$ /~user/_site/$1 [L]