So my viewport just wont switch to fullscreen. Neither F11 or shift + F11 or ALT + enter is working 
i read that the viewport has to be “docked” to then switch it in fullscreen but that awso just dont work. Is “docked” viewport mean that the viewport is not floating solo on the screen so like holding left mouse on it and drag it to the top or left side of the screen for example then orange lines appear and it sticks then to the side when i move it to those corners. Is this the so called “docked” viewport mode ? If yes then this dont work as well for me it still wont go in fullscreen by pressing f11 or shift + f11. Other solution i read was closing background programms that may interfere with it, but that didnt help as well. So pressing on the play button has same result F 11 or shift +F11 do nothing. It feels bad and is really frustrating if i cant fix at least this because my movie render q wont work as well on dx 12 and i was thinking as no other choice to render my scene without it i could at least start my sequence in fullscreen then record it with external tool that can record my screen but i cant even do that now till fullscreen wont work. Is really frustrating to work very hard on a scene a long time and at the end i cant render it. only solution is movie render q on dx 11 which works but my whole geometry is broken with wierd artefacts on the meshes so this is a no option to me and i need dx 12 or vulkan. Here is my other post to the movie render q if someone have idea what i can do there to fix that. Movie Render Q working on dx 11 but not on dx 12? - #2 by catz111
I just found out a solution which acctually works 
. So by pressing F11 it still does nothing but by clicking on the top left corner on the 3 line symbol then selecting “immersive mode” it do switch my viewport to fullscreen. So this looks like finally at least i can use it now. No hotkey to do it but at least manualy by clicking there does the trick. i guess problem is now solved even if its not 100% working as intended with a hotkey
the reason is, that the Viewport and PIE Modes are Editor internal and so they are not seperated Windows.
by using standalone or shipped build, the Hotkey works, cause they are self running executables, handled by the OS.
But you can use “SetFullscreenMode” in PIE, too. Just create an input binding that fakes your F11 key while developing.