*Shortly:* The reason was that 'guac_client.onpipe' check (in Client.js) appears before my onpipe declaration.
*Details:* There are two event listeners /$window.addEventListener('load', checkClipboard, true); $window.addEventListener('focus', function focusGained(e) { // Only recheck clipboard if it's the window itself that gained focus if (e.target === $window) checkClipboard(); }, true);/ In IE11 two MessageBoxes appear: first on 'load' event (before my 'onpipe' declaration), the second on 'focus' event (before 'onpipe' check in Client.js). So second MessageBox overlapped the first one, so the user closes it first ('onpipe' check passes) and only then user closes the first one ('onpipe' declaration succeeded, but it is too late). For my application I removed the event /$window.addEventListener('load', checkClipboard, true); / In any case 'checkClipboard' is called on 'focus' event I am testing it now... -- Sent from: http://apache-guacamole-general-user-mailing-list.2363388.n4.nabble.com/