Hi,
It's currently impossible to specify a vertical separator between buttons in OfficeJS defined controls. It's a common way to improve UI readability and was available on other Office addon development platforms. Please consider supporting it.
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There's also a similar request submitted for a separator in office js plugin menus that should be considered along with this request. https://techcommunity.microsoft.com/t5/microsoft-365-developer-platform/office-js-context-menu-does-not-provide-separators/idc-p/3914982
