When the Editor is configured to occupy size in percentages and the user switches from default to full screen mode and vice versa, the content area of the control gets broken. The problem is reproducible in all browsers.

Video: screencast



As a temporary workaround you can attach a handler to the Editor's ClientCommandExecuted event and repaint the Editor in case the current command is ToggleScreenMode:

        <div style="width: 700px; height: 500px">
            <telerik:RadEditor ID="RadEditor1" runat="server" Width="100%" Height="100%" OnClientCommandExecuted="OnClientCommandExecuted">
                <Content>
                <p>dsadasdasdad dasdasdd das dasdasdasd <br />
                    dsadasdasdad dasdasdd das dasdasdasd <br />
                    dsadasdasdad dasdasdd das dasdasdasd <br />
                    dsadasdasdad dasdasdd das dasdasdasd <br />
                    dsadasdasdad dasdasdd das dasdasdasd <br />
                </p>
                </Content>
            </telerik:RadEditor>
        </div>
        <script>
            function OnClientCommandExecuted(editor, args) {
                if (args.get_commandName() == "ToggleScreenMode") {
                    editor.repaint();
                }
            }
        </script>