You can open the file:
/plugins/system/t3/admin/layout/css/layout.css
Line 388:
.t3-off-canvas {
position: absolute;
right: -240px;
top: 0;
width: 220px;
background: #f2f2f2;
padding: 0 12px;
}
change it to:
.t3-off-canvas {
position: relative;
right: 0;
top: 0;
width: 220px;
background: #f2f2f2;
padding: 0 12px;
}