Adjustable width in split view

[cosimo193]

I've been using another email app for a long time now, but would like to use Thunderbird, however there are a few little things that are annoying about the current version compared to my existing app. 

I would like to be able to make the left pane in the split view narrower. I have tried, but don't see an option or anything to tap and hold to do that. Attached is a screenshot of thunderbird vs my current app; there's only a small difference but, aesthetically, to me, it is substantial because of the proportions. 

[Jon]

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