Reproducing:

• Open demo sample
• Go to sandbox tab
• Paste this in rml box:

<div />

• Paste this in rcss box:

div
{
width: 100%;
height: 100%;
background-color: red;
position: relative;
left: 100%;
}

• It should look like this:
  
• Make the window full screen. The red box should remain aligned with the right of white boxes, but instead it looks something like this:
  

Resizing the window also exhibits the bug but it's less visible.