trilium/src/public/app/widgets/buttons/left_pane_toggle.js

import ButtonWidget from "./button_widget.js";
import options from "../../services/options.js";
import splitService from "../../services/resizer.js";

export default class LeftPaneToggleWidget extends ButtonWidget {
    refreshIcon() {
        const isLeftPaneVisible = options.is('leftPaneVisible');

        this.settings.icon = isLeftPaneVisible
            ? "bx-chevrons-left"
            : "bx-chevrons-right";

        this.settings.title = isLeftPaneVisible
            ? "Hide panel."
            : "Open panel.";

        this.settings.command = isLeftPaneVisible
            ? "hideLeftPane"
            : "showLeftPane";

        super.refreshIcon();

        splitService.setupLeftPaneResizer(isLeftPaneVisible);
    }

    entitiesReloadedEvent({loadResults}) {
        if (loadResults.isOptionReloaded("leftPaneVisible")) {
            this.refreshIcon();
        }
    }
}