element-web/res/css/structures/_MainSplit.scss

/*
Copyright 2019 New Vector Ltd

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

.mx_MainSplit {
    display: flex;
    flex-direction: row;
    min-width: 0;
    min-height: 0;
    height: 100%;
}

.mx_MainSplit > .mx_RightPanel_ResizeWrapper {
    padding: 5px;
    // margin left to not allow the handle to not encroach on the space for the scrollbar
    margin-left: 8px;
    height: calc(100vh - 51px); // height of .mx_RoomHeader.light-panel

    &:hover .mx_RightPanel_ResizeHandle {
        // Need to use important to override element style attributes
        // set by re-resizable
        top: 50% !important;
        transform: translate(0, -50%);

        height: 64px !important; // to match width of the ones on roomlist
        width: 4px !important;
        border-radius: 4px !important;

        background-color: $primary-content;
        opacity: 0.8;
    }
}