Trilium/public/javascripts/status.js

function checkStatus() {
    $.ajax({
        url: baseApiUrl + 'status',
        type: 'POST',
        contentType: "application/json",
        data: JSON.stringify({
            treeLoadTime: globalTreeLoadTime,
            currentNoteId: globalCurrentNote ? globalCurrentNote.detail.note_id : null,
            currentNoteDateModified: globalCurrentNoteLoadTime
        }),
        success: resp => {
            if (resp.changedTree) {
                loadTree().then(resp => {
                    console.log("Reloading tree because of background changes");

                    globalTree.fancytree('getTree').reload(resp.notes);
                });
            }

            if (resp.changedCurrentNote) {
                alert("Current note has been changed in different window / computer. Please reload the application and resolve the conflict manually.");
            }

            $("#changesToPushCount").html(resp.changesToPushCount);
        },
        statusCode: {
            401: () => {
                // if the user got logged out then we should display the page
                // here we do that by reloading which will force the redirect if the user is really logged out
                window.location.reload(true);
            },
            409: () => {
                // 409 means we need to migrate database, reload will take care of it
                window.location.reload(true);
            }
        }
    });
}

setInterval(checkStatus, 5 * 1000);