Fullscreen Slider Startpage

body .inside-page-header-container.grid-container {
        position: absolute;
        top: 50%;
        left: 50%;
        margin-right: -50%;
        transform: translate(-50%, -50%)
}

http://cyrillucidokuhn.com/slider-header-content/
Like this it almost works out... :D
When I open the page the first time it's not centred, but if I change short the page proportions or reload the page it gets sometimes centred!?

Looks centered to me on first load: https://www.screencast.com/t/6pTgk4jAUR5N

Yes I somehow manage to center it now on first load, but when I reload the page or minimise and maximise the browser it goes out of center. I don't really get why this happens...

- deleted -

body .site.grid-container {
        position: absolute;
        top: 50%;
        left: 50%;
        margin-right: -50%;
        transform: translate(-50%, -50%);
}

http://cyrillucidokuhn.com/slider-page-content/

here the same problem occurs.

Here's what I see: https://www.screencast.com/t/IBhBLbDC0Mo

Thank you Tom!

It's strange, here is what I see:
https://www.screencast.com/t/Q4bip9r5N

After reloading and minimising maximising it looks then like yours...

What browser?

Safari

Thanks in advance. I am leaving today on a trip for roughly 2 weeks - 1 month so I will take time to respond.

Best wishes
Yannick

Can you try this?:

body .site.grid-container {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}