Custom css modification for Vivaldi Browser to auto hide vertical tab bar. Suitable for workspace selector in tab bar. Fixed for full screen elements like that in youtube as well. Based on (https://www.reddit.com/r/vivaldibrowser/comments/1b7uh4v/guide_resizing_vertical_tabs_on_hover/)

common.css

/* Expanding Left Tabs */

/* Animate the tabs, set initial width. */
#tabs-tabbar-container.left {
    width: inherit !important;
}

.tabbar-wrapper {
    position: absolute !important;
    z-index: 200 !important;
    height: 100% !important;
    transition: all 250ms ease !important;
    width: 32px;
}

.tabbar-wrapper:hover {
    width: 250px !important;
}

#webview-container {
    margin-left: 32px;
}

@media all and (display-mode: fullscreen) {
    /* For full screen inside element like youtube, facebook videos */
    .inner>#main>.inner>#webview-container {
        margin-left: 0 !important;
    }
}

/* Shunt the status info (text on hover) over to accomodate tabs */
#webview-container ~ .StatusInfo {
    left: 36px !important;
}

.button-popup[aria-label="Workspaces"] {
    z-index: 300 !important;
}

.tabbar-workspace-button {
    width: inherit !important;
}

/* To hide workspace extra text and dropdown chevron */
.tabbar-wrapper>.tabbar-workspace-button>.ToolbarButton-Button {
    justify-content: flex-start !important;
    overflow: hidden;
}