When a RadEditor's toolbar mode is set to RibbonBar, its left and right border disappear. 

Possible solutions are to set custom CSS styles that fixes the resizing issue under IE11:

Approach 1:
.RadRibbonBar.RadRibbonBar_<SkinName> {
 margin:1px;
}

Approach 2:
.reToolbarWrapper {
 padding-left:1px;
 padding-right:1px;
}