web/web_chatter_position/static/src/scss/attachment_viewer.scss

// Attachment Viewer
.o_web_client.o_chatter_position_sided .o_DialogManager_dialog {
    /* Show sided viewer on large screens */
    @include media-breakpoint-up(lg) {
        position: static;
        .o_AttachmentViewer_main {
            padding-bottom: 20px;
        }
        .o_AttachmentViewer {
            // On-top of navbar
            z-index: 10;
            position: absolute;
            right: 0;
            top: 0;
            bottom: 0;
            margin-left: auto;
            background-color: rgba(0, 0, 0, 0.7);

            width: $chatter_zone_width;
            &.o_AttachmentViewer_maximized {
                width: 100%;
            }

            /* Show/Hide control buttons (next, prev, etc..) */
            &:hover .o_AttachmentViewer_buttonNavigation,
            &:hover .o_AttachmentViewer_toolbar {
                display: flex;
            }
            .o_AttachmentViewer_buttonNavigation,
            .o_AttachmentViewer_toolbar {
                display: none;
            }
            .o_AttachmentViewer_viewIframe {
                width: 95%;
            }
        }
    }
    @include media-breakpoint-down(md) {
        .o_AttachmentViewer_headerItemButtonMinimize,
        .o_AttachmentViewer_headerItemButtonMaximize {
            display: none;
        }
    }
}

/* Attachment Viewer Max/Min buttons only are useful in sided mode */
.o_web_client:not(.o_chatter_position_sided) {
    .o_AttachmentViewer_headerItemButtonMinimize,
    .o_AttachmentViewer_headerItemButtonMaximize {
        display: none;
    }
}