There’s a weird issue around Safari that I’ve encountered previously with items dropping. Shave .01% off the width and it seems to fix it, so for your desktop, try 24.9% as the width and see how you get on.
I will try and look more into that and find out why it happens though.
For the white content container, you have the height set as 100%, but there is nothing explicitly set to say what it should be 100% of, and Safari has a real issue with this. The parent has 100% height, but the parent of THAT needs to be told some height measurements so that they know how high 100% is.
(Think of it like me saying to you I want 50% of some rope, but the rope has an undefined length… what is 50%?)
Please let me know if that helps at all!