wp-calypso/client/post-editor/media-modal/content.scss

.editor-media-modal__content {
	display: flex;
	flex-direction: column;
	position: absolute;
	top: 52px;
	right: 0;
	bottom: 74px;
	left: 0;
	overflow-y: auto;

	@include breakpoint-deprecated( ">660px" ) {
		flex-direction: row;
		top: 74px;
		overflow-y: visible;
	}
}