bootstrap-table/src/extensions/fixed-columns/bootstrap-table-fixed-columns.scss

.fixed-columns,
.fixed-columns-right {
  position: absolute;
  top: 0;
  height: 100%;
  background-color: #fff;
  box-sizing: border-box;
  z-index: 1;
}

.fixed-columns {
  left: 0;

  .fixed-table-body {
    overflow: hidden!important;
  }
}

.fixed-columns-right {
  right: 0;

  .fixed-table-body {
    overflow-x: hidden!important;
  }
}