RH2019 How do you keep entire TOC tree when expanding a folder?

I did some further poking around and it looks like Classic has a setting for the mobile toc that allows you to select whether the toc expands or drills down (and it's set to expand by default). But it looks like Azure Blue in New UI is configured to drill down, and there is no option to change it, that I can see. 

It's occurred to me you might be seeing one of the effects of a responsive layout. Depending on the width of your browser window, the help will automatically switch to tablet or phone mode. If you see icons arranged vertically in a narrow bar, then you are viewing your output in tablet mode. In tablet mode, clicking a book, will drill down into the book rather than expanding.

Make your browser wider and see if that displays as you expect. 

I sent my project as instructed. There has been no communication of topics between 2019 classic and new UI.  I input new topics into the project I sent.  Iam still having the issues with ResponsiveHTML5 output. Thank you so much for your help.

Create a new project in 2019 New UI with just a few topics and a TOC with a couple of books. Test the problem occurs there. If it does, then see the Contact page on my site and send the project as instructed there. Do make sure you include a link to this thread and please do not email the project direct.

If it doesn't happen in the new project, then the problem is within your project. Can you confirm that there has not been any exchange of topics between RoboHelp Classic and the new UI version of 2019?

Yes.  I just found that.  I gave it a new name Azure_Blue_(RTL)2. I get the same behavior....the TOC displays with the folder structure, but when I open a folder, the TOC pane overwrites to display just that folder/topics.

I downloaded the RoboHelp 2017 trial and the TOC displays just fine using the Azure Blue skin for a Responsive HTML5 ouput.  I can open a folder and the rest of the folders remain displayed as well.  I may be going with RoboHelp 2017. (And, yes, I tried just the "Azure Blue" skin with RoboHelp 2019 (both versions) for Responsive HTML5 output and the TOC will not retain an open tree structure when any of the folders are selected.)

See your screenshot, Skins are just below Output Presets. Click + and select your preferred skin. If you want Azure Blue RTL again, give it a slightly different name.

What do you mean by create a new skin?  I can only select skins in the drop-down. I assume that I can just copy/rename the skin in the skin folder.  (Where is the skin folder?)

That is not what I am seeing using an Azure Blue RTL with no mods.

Create a new skin and add say 2 at the end of its name. Try again. Still doing the same?

I am talking about what the user sees.  In my Responsive HTML5 output, the TOC shows the folders collapsed.  When I open the first folder, the folder expands and I see its topics, but that's all I see...the entire TOC is no longer displayed.