Memory leaks happen when opening and closing emails in new windows (by double-clicking) with pinnable add-in with Edge Webview. The memory usage of Desktop App Web Viewer process grows to an extent that it might freeze Outlook or reaches the add-in memory limit and crashes ultimately, rendering add-ins unusable.
Microsoft has provided a fix for this issue with the August 2020 Microsoft update for Windows users of 1903 and 1909 (see this article from Microsoft for details).
Although Microsoft has acknowledged they have fixed the issue we are keeping the detail below for historical context.
📌 This was a known Microsoft issue tracked by them on GitHub: https://github.com/OfficeDev/office-js/issues/1069
Users with Microsoft add-ins such as CloudExtend running in Edge Webview. Beginning with Office 365 version 16.0.11629 and Windows 10 version 1903, Office Add-ins running on Office 365 for Windows will use a Microsoft Edge WebView as the runtime. The selection of Edge WebView is entirely decided by Microsoft.
How to reproduce
Step 1: Open Outlook desktop app and log in.
Step 2: Open an email, and open a pinnable add-in (CloudExtend Outlook for NetSuite in this case).
Step 3: Pin the task pane.
Step 4: Open Task Manager.
Step 5: Double click an email in the inbox, wait for it to open in a new window, and wait for the pinnable add-in to load.
Step 6: Close the new window.
Step 7: Repeat Steps 4 and 5 multiple times.
Step 8: The memory usage of Desktop App Web Viewer keeps growing at a significant rate.
Microsoft recommends users install the August 2020 update (see here).
If you are unable to install the update then choose one of the below two options
If keeping the task pane pinned is important to you then periodically open Task Manager (Ctrl-Shift-Esc), find Desktop App Web Viewer processes
Right-click → End task (Temporary workaround when freezing happens).
Do not pin the add-in (suggested by Microsoft).
How to End task the Desktop App Web Viewer