The popout window will by default take the name of the file, which is “file”. If you would be able to change the source code of the FileDocumentViewer widget you could implement something like this, to set the window title to the document name after the window was opened.
I made this change in the file: FileDocumentViewer.js
_eventPopout: function () {
logger.debug(this.id + "._eventPopout");
//window.open(this._getFileUrl());
//Add by Jeroen
var w = window.open(this._getFileUrl());
var title = 'File';
if (this._contextObj && this._contextObj.get("HasContents")) {
if (this.escapeTitle) {
title = dom.escapeString((this._contextObj.get(this.headertitle) || "").replace(/\n/g, " "));
} else {
title = this._contextObj.get(this.headertitle);
}
}
setTimeout(() => w.document.title = title, 0);
//End added by Jeroen
},