riot-web/res/css/structures/_LeftPanelWidget.scss

/*
Copyright 2020 The Matrix.org Foundation C.I.C.

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_LeftPanelWidget {
    // largely based on RoomSublist
    margin-left: 8px;
    margin-bottom: 4px;

    .mx_LeftPanelWidget_headerContainer {
        display: flex;
        align-items: center;

        height: 24px;
        color: $tertiary-content;
        margin-top: 4px;

        .mx_LeftPanelWidget_stickable {
            flex: 1;
            max-width: 100%;

            display: flex;
            align-items: center;
        }

        .mx_LeftPanelWidget_headerText {
            flex: 1;
            max-width: calc(100% - 16px);
            line-height: $font-16px;
            font-size: $font-13px;
            font-weight: 600;

            // Ellipsize any text overflow
            text-overflow: ellipsis;
            overflow: hidden;
            white-space: nowrap;

            .mx_LeftPanelWidget_collapseBtn {
                display: inline-block;
                position: relative;
                width: 14px;
                height: 14px;
                margin-right: 6px;

                &::before {
                    content: '';
                    width: 18px;
                    height: 18px;
                    position: absolute;
                    mask-position: center;
                    mask-size: contain;
                    mask-repeat: no-repeat;
                    background-color: $tertiary-content;
                    mask-image: url('$(res)/img/feather-customised/chevron-down.svg');
                }

                &.mx_LeftPanelWidget_collapseBtn_collapsed::before {
                    transform: rotate(-90deg);
                }
            }
        }
    }

    .mx_LeftPanelWidget_resizeBox {
        position: relative;

        display: flex;
        flex-direction: column;
        overflow: visible; // let the resize handle out
    }

    .mx_AppTileFullWidth {
        flex: 1 0 0;
        overflow: hidden;
        // need this to be flex otherwise the overflow hidden from above
        // sometimes vertically centers the clipped list ... no idea why it would do this
        // as the box model should be top aligned. Happens in both FF and Chromium
        display: flex;
        flex-direction: column;
        box-sizing: border-box;

        mask-image: linear-gradient(0deg, transparent, black 4px);
    }

    .mx_LeftPanelWidget_resizerHandle {
        cursor: ns-resize;
        border-radius: 3px;

        // Override styles from library
        width: unset !important;
        height: 4px !important;

        position: absolute;
        top: -24px !important; // override from library - puts it in the margin-top of the headerContainer

        // Together, these make the bar 64px wide
        // These are also overridden from the library
        left: calc(50% - 32px) !important;
        right: calc(50% - 32px) !important;
    }

    &:hover .mx_LeftPanelWidget_resizerHandle {
        opacity: 0.8;
        background-color: $primary-content;
    }

    .mx_LeftPanelWidget_maximizeButton {
        margin-left: 8px;
        margin-right: 7px;
        position: relative;
        width: 24px;
        height: 24px;
        border-radius: 32px;

        &::before {
            content: '';
            width: 16px;
            height: 16px;
            position: absolute;
            top: 4px;
            left: 4px;
            mask-position: center;
            mask-size: contain;
            mask-repeat: no-repeat;
            mask-image: url('$(res)/img/feather-customised/maximise.svg');
            background: $muted-fg-color;
        }
    }
}

.mx_LeftPanelWidget_maximizeButtonTooltip {
    margin-top: -3px;
}