Why is PHPStorm (flatpak) unable to access /var/www?

Question

A short introduction about myself
I have just installed Feren OS. As a webdeveloper I do work with Linux and Unix commandline on a daily basis, but only for quite simple stuff. So I still consider myself a Linux noob.

Edit: This question has now been downvoted once. Please consider to let me know what is wrong with it, if you downvote it.

My problem
First of all I have installed Apache2 from commandline (using apt). I have created a fully functioning working website in /var/www/projects/some_website/web. This part is what I am familiar with for my daily job as well.

I have also installed PHPStorm (flatpak) from the store. The store itself is (according to it's about box) a Feren OS custom made store. However, I'd say that has nothing to do with the problem I'm having.

In Feren OS I am logged in as user "paul".

Now when - as user "paul" - I open up the terminal, I can go anywhere. For instance I can go to "/var/www". I can create directories there, chown them, etc. Like I said, this part I am familiar with.

However from PHPStorm I can go to "/var", but there I can't see all the directories. Even when I type "/var/www" in the address box there, it can't go there.

As a comparison I have also opened text editor Kate. This program can access "/var/www" like I would expect it to.

What I tried to fix it
The answers I found on the internet mentioned mainly that the user should be added to the www-data group and /var/www/ should be chowned by that group.

For example this one

While I consider this solution nonsense in my case, I did try it. I consider it nonsense, because other applications running under the same user can access the directory. Ofcourse I can be in the wrong here, so please let me know :)

To be sure I have added user "paul" to the "www-data" group and I have chowned /var/www to that group as you can see in the attached screenshot.

Another possible solution I found is installing PHPStorm via snap instead. I haven't tried that yet, because I would like to understand what I doing wrong first.

Last but not least I have uninstalled the flatpak package in the Feren OS store and then re-installed the flatpak package from commandline:

flatpak install PhpStorm

This also does not change anything.

My questions
1. What is the difference between PHPStorm and Konsole and Kate, that makes that PHPStorm cannot access /var/www/, while all the other tools can.

2. What should I do to fix this problem?

Investigation

As asked in the comments I have checked what user is running PHPStorm:

ps -ef | grep -i phpstorm

paul      2241  1332  0 13:09 ?        00:00:00 /usr/libexec/flatpak-bwrap --args 31 phpstorm
paul      2267  2241  0 13:09 ?        00:00:00 /usr/libexec/flatpak-bwrap --args 31 phpstorm
paul      2268  2267  0 13:09 ?        00:00:00 /bin/sh /app/extra/phpstorm/bin/phpstorm.sh
paul      2309  2268 99 13:09 ?        00:00:19 /app/extra/phpstorm/jbr/bin/java -classpath /app/extra/phpstorm/lib/bootstrap.jar:/app/extra/phpstorm/lib/extensions.jar:/app/extra/phpstorm/lib/util.jar:/app/extra/phpstorm/lib/jdom.jar:/app/extra/phpstorm/lib/log4j.jar:/app/extra/phpstorm/lib/trove4j.jar:/app/extra/phpstorm/lib/jna.jar -Xms128m -Xmx968m -XX:ReservedCodeCacheSize=240m -XX:+UseConcMarkSweepGC -XX:SoftRefLRUPolicyMSPerMB=50 -ea -XX:CICompilerCount=2 -Dsun.io.useCanonPrefixCache=false -Djdk.http.auth.tunneling.disabledSchemes="" -XX:+HeapDumpOnOutOfMemoryError -XX:-OmitStackTraceInFastThrow -Djdk.attach.allowAttachSelf=true -Dkotlinx.coroutines.debug=off -Djdk.module.illegalAccess.silent=true -Dawt.useSystemAAFontSettings=lcd -Dsun.java2d.renderer=sun.java2d.marlin.MarlinRenderingEngine -Dsun.tools.attach.tmp.only=true -XX:ErrorFile=/home/paul/java_error_in_PHPSTORM_%p.log -XX:HeapDumpPath=/home/paul/java_error_in_PHPSTORM.hprof -Didea.paths.selector=PhpStorm2020.1 -Djb.vmOptionsFile=/home/paul/.var/app/com.jetbrains.PhpStorm/config/JetBrains/PhpStorm2020.1/phpstorm64.vmoptions -Didea.platform.prefix=PhpStorm com.intellij.idea.Main
paul      2403  2387  0 13:09 pts/1    00:00:00 grep --color=auto -i phpstorm

And as a comparison I have done the same for Kate (which can access the directory):

ps -ef | grep -i kate

paul      2761  1332  4 13:12 ?        00:00:00 /usr/bin/kate -b --tempfile
paul      2767  1271  0 13:12 ?        00:00:00 tags.so [kdeinit5] tags local:/run/user/1000/klauncherkWHCgQ.1.slave-socket local:/run/user/1000/kateencBio.1.slave-socket
paul      2776  1271  0 13:12 ?        00:00:00 file.so [kdeinit5] file local:/run/user/1000/klauncherkWHCgQ.1.slave-socket local:/run/user/1000/kateSKaSMP.2.slave-socket
paul      2780  1271  0 13:12 ?        00:00:00 thumbnail.so [kdeinit5] thumbnail local:/run/user/1000/klauncherkWHCgQ.1.slave-socket local:/run/user/1000/kateVcVjeG.4.slave-socket
paul      2791  2387  0 13:12 pts/1    00:00:00 grep --color=auto -i kate

Answer 1

The developer of Feren OS told me that the problems most likely occur because of the permissions of the Flatpak package "user".

Then the JetBrains support desk told me something similar and advised to use Snap or the JetBrains Toolbox instead to install PHPStorm. I have now installed PHPStorm via their toolbox and it runs like a charm.