Looks like a regression from some de-XBL activity.

For me Subject and also Date is missing in the standalone window. In a tab and message preview pane in 3-pane window all fields look OK.

The reason behind missing subject field is that the custom element is not registered when headerValue of mail-headerfield is changed. https://searchfox.org/comm-central/source/mail/base/content/msgHdrView.js#1007 Magnus, I will need your help here for making sure that mail-headerfield custom element is into scope when new window is opened. We probably have to find a foolproof way of letting the custom element definition in scope before any window loads. I am attaching screenshot to confirm what I am saying.

customElements.get(node.nodeName) or node instanceof MozXULElement, makes sure that node is custom elment node and not a normal xul node.

Needed to load the customElements.js earlier.

Comment on attachment 9016559 [details] [diff] [review] bug1497656_standalone_win_custom_elements.patch Review of attachment 9016559 [details] [diff] [review]: ----------------------------------------------------------------- Thanks, works for me.

Pushed by mozilla@jorgk.com: https://hg.mozilla.org/comm-central/rev/e2b203306dad set up Thunderbird custom elements on DOMWindowCreated, to make them work also in the stand-alone message window. r=aceman