Run gnome-tweaks and click on the "Window Titlebars" menu-entry. There should be an option for you to enable "Maximize" and "Minimize" buttons".

These are disabled by default. The combination of Workspaces and using the "Activities" layout to switch between windows is used by many instead of the old-style "minimize to the taskbar".

I suspect some programs still have the minimize and maximize buttons because they draw their own title-bars and don't check system settings. I can confirm that chromium and MS Teams behave like that.

It looks like that the problem is solved by an another update (I am not experiencing this issue anymore). Nevertheless, thanks for tips.

The Form has two properties called MinimizeBox and MaximizeBox, set both of them to false.

To stop the form closing, handle the FormClosing event, and set e.Cancel = true; in there and after that, set WindowState = FormWindowState.Minimized;, to minimize the form.

You can do that using Windows themes, with which you may customize just about every aspect of Windows.

However, this is a complex subject, and creating themes as XML files is not for everyone, so using a third-party product is recommended.

Perhaps the best product in that area is Stardock WindowBlinds, which is however trialware (although not very costly).

Well I managed to solve this only because of lucky.

I was searching for the same answer. First I disabled the menus from the windows doing the commands:

sudo apt-get autoremove appmenu-gtk appmenu-gtk3 appmenu-qt indicator-appmenu

Then I restarted the pc and then I went to Appearance Configuration > Behavior (I am using portuguese version so I dont know if this is the correct translation). And then there is a check box "Show menus" on "the title bar".

That solved the problem for me.

You can't hide it, but you can disable it by overriding the CreateParams property of the form.

private const int CP_NOCLOSE_BUTTON = 0x200;
protected override CreateParams CreateParams
{
    get
    {
       CreateParams myCp = base.CreateParams;
       myCp.ClassStyle = myCp.ClassStyle | CP_NOCLOSE_BUTTON ;
       return myCp;
    }
}

Source: http://www.codeproject.com/KB/cs/DisableClose.aspx

This is a browser limitation - you can't do this. Malicious actions like these are disabled by browsers.

However, you can open a window without a toolbar or menubar:

window.open("mypage.html","mywindowname", "toolbar=no,menubar=no");

Check out this reference for more information on window.open.