Wow, but I see what you're saying.
I have an on browser resize event that calls a function to handle the resizing. I also call that function on the start of any layout.
I did some testing and put the ViewportLeft and ViewportWidth into a text box just before it called the function. Well into the function is where I resize one of the HTMLElements and I grabbed the ViewportLeft and Width again and put them in a different text box. If I slowly resized I was seeing maybe a 0.5 - 3px difference. But, if I did a quick drag and release resize, I was seeing 50-100px differences.
If I clicked on the full-screen request button, the px differences were pretty huge, like 500px+
Now, whether this is affecting that CSS div, I dunno.