[MlMt] big sur, new message view window issue

[moiraine]

already reported in bug tracker, current I do not have a non-big sur
machine to test if it's a mailmate issue or big sur issue: On build Version
1.13.2 (5720), using the new message view, after viewing a couple of
messages, mailmate will spawn multiple webview processes that do not quit
(e.g. seen by searching for "mailmate" in the activity monitor, showing
multiple processes titled "about:"); in terminal, the corresponding pid for
the "about:" processes in the attached screenshot is

```
2541 ??         0:00.34
/System/Library/Frameworks/WebKit.framework/Versions/A/XPCServices/com.apple.WebKit.WebContent.xpc/Contents/MacOS/com.apple.WebKit.WebContent
```

link:
https://freron.lighthouseapp.com/projects/58672/tickets/2646-message-view-spawning-multiple-processes-that-does-not-quit#ticket-2646-1

Thanks!
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