When you resize the RadEditor beyond the boundaries of the browser's viewport, absolute positioning will be applied to the control, which could result in problems with the layout in certain cases.

You can use the following workaround to fix the positioning of the editor until the problem is resolved. Just add the script below at the end of your page:

<script type="text/javascript">
                var onResizeStart = Telerik.Web.UI.RadEditor.prototype.onResizeStart;
                Telerik.Web.UI.RadEditor.prototype.onResizeStart = function() {
                                var editorWrapper = this.get_element();
                                this.resizeStartPosition = {
                                                position: editorWrapper.style.position,
                                                top: editorWrapper.style.top,
                                                left: editorWrapper.style.left
                                };
                                onResizeStart.call(this);
                }

                var onResizeEnd = Telerik.Web.UI.RadEditor.prototype.onResizeEnd;
                Telerik.Web.UI.RadEditor.prototype.onResizeEnd = function() {
                                var editorWrapper = this.get_element();
                                if (this.resizeStartPosition && this.resizeStartPosition.position != editorWrapper.style.position) {                                             
                                                $telerik.$(editorWrapper).css({
                                                                position: this.resizeStartPosition.position,
                                                                top: this.resizeStartPosition.top,
                                                                left: this.resizeStartPosition.left
                                                });
                                }
                                onResizeEnd.call(this);
                }
</script>