Expanded Folders
Listed under "WHAT'S NEW?" is this:
"Expanded Folders - Now you can keep them collapse/uncollapse if you want your Left Rail less cluttered."
Every comment I've seen complaining about folders has a response from an admin stating you can "click on the down arrow next to folders to expand them."
There is no arrow. There is no way to keep folders expanded. Yahoo Mail seems to be getting worse not better.

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Hi James and Donny, we are going to email you and get your screenshot! If you're not seeing the drop down arrow, this is indeed a bug. Thanks for letting us know!
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Donny Conway commented
This option is not there for me. I tried to post a screen shot but I have a 'folder' on the left pannel and when you click it a tiny tiny box comes up that you can endless scroll thru ... that's it ... there is no option to collapse/uncollapse?