I tried dragging in a folder of HTML and JS files into the sidebar. What happens is that most of the files are uploaded empty. Meaning the file name is there, but it’s empty. Sometimes the editor will stay frozen with the previous viewed file and whichever recent uploaded file you click it won’t open.
Try uploading some dummy HTML files by multi-selecting them, then drag & drop into the sidebar. Obviously I’m not going to provide a ZIP file of dummy files for security reasons (for your own sake).