How to remove IDEA's titlebar?

See Appearance & Behavior → Appearance Settings page

Solution for KDE Users:
1- Just open a text editor and enter these lines and save it as no_titlebar_for_intellij_idea.kwinrule

The goal isn't to remove the titlebar without window controls, the goal is to remove the titlebar and have the window controls embedded in the top menu the way Firefox does, as shown in this comment: 

https://intellij-support.jetbrains.com/hc/en-us/community/posts/4479475576850/comments/4490531760914


"Preferences | Appearance & Behavior | Menus and Toolbars" check the "Main menu" and "Main Toolbar" here to remove needed elements. 

No, that's the Main Toolbar, not the Titlebar of the Window.  The thing that the windowing system gives windows unless the window requests an "undecorated window".

This top rectangle that is showing the project name and currently open java file:

For example, here is how Firefox does it.  Note, the File Menu (Main Toolbar for you) is on the same line as the minimize/maximize/close buttons.

(This Firefox is on Linux Mint / MATE):

Is this a new thing?  I'm using 2021.3 (fairly recent), and I don't think it's in the settings UI.  Searching for merge or title bar doesn't yield it either.

This is what I'm seeing (machine is Linux Mint MATE):

IntelliJ IDEA 2021.3 (Community Edition)
Build #IC-213.5744.223, built on November 27, 2021
Runtime version: 11.0.13+7-b1751.19 amd64
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o.
Linux 5.8.0-63-generic
GC: G1 Young Generation, G1 Old Generation
Memory: 2000M
Cores: 4
Registry:
    documentation.show.toolbar=true

Non-Bundled Plugins:
    com.jetbrains.plugins.ini4idea (213.5744.190)

Kotlin: 213-1.5.10-release-949-IJ5744.223
Current Desktop: MATE

Sorry for the misunderstanding. The feature is being implemented. You can track the status at https://youtrack.jetbrains.com/issue/IDEA-199526

Solution for KDE Users:
1- Just open a text editor and enter these lines and save it as no_titlebar_for_intellij_idea.kwinrule

[No Titlebar for Jetbrains Idea]
Description=No Titlebar for Jetbrains Idea
noborder=true
noborderrule=2
wmclass=jetbrains-idea
wmclassmatch=1

2- just enter this command in any terminal [ konsole, yakuake, ... ] systemsettings kcm_kwinrules
3- and click import and select `no_titlebar_for_intellij_idea.kwinrule` [ create it like i mentioned above ]
Thats all now just run idea

[ you dont need to repeat these steps everytime]