silverstripe-framework/admin/javascript/src/styles/_layout.scss

.content-toolbar {
	max-height: $navbar-height;
    display: block;
    padding: 10px 0 10px 12px;
    margin-bottom: $spacer-y/2;

		.btn-toolbar {
			margin-top: 0;
			margin-bottom: 0;
		}
}

.cms-middle {
	padding-left: 0;
	height: 100%;
	position: relative;
	transition: padding .2s;

	&.with-preview {
		@media (min-width: 992px) { /* lg */
			padding-left: 316px;
			.cms-campaigns {
				width: 316px;
			}
		}
		@media (min-width: 1200px) { /* xl */
			padding-left: 448px;
			.cms-campaigns {
				width: 448px;
			}
		}
	}
}

.cms-middle__scrollable {
	overflow-y: auto;
	display: block;
	height: 100%;
}

// Preview panel
.pages-preview {
	display: block;
	position: relative;
	background-color: #BBB;
	border-left: 1px solid #dbdde0;
	height: 100%;

	iframe {
		width: 100%;
		height: calc(100% - #{$navbar-total-height});
		border: none;
	}
}

.cms-south-actions {
    height: $navbar-total-height;
    position: absolute;
    bottom: 0;
    width: 100%;
    border-top: 1px solid $border-color;
    background-color: $body-bg;
    padding: $spacer-y*.625 $spacer-x;

	.btn-toolbar {
		// TODO Remove '.cms .btn-toolbar' override
		margin-top: 0;
		margin-bottom: 0;
	}
}