While using Thunderbird, I clicked on the left somewhere, and all of a sudden my folders and my mini-calendar were gone. What happened? I attempted to click and drag on the very left edge where the edge of the window was, but that didn't work.
Tried all the options in the view menu, looked for other menus. Nothing.
Googled it, and came up with nothing.
Went back into Thunderbird, tried again. This time, it worked. My folder list and calendar re-appeared.
BFI sometimes works. You just have to try harder.
The reason this happened is that the resize is "sticky". You can make the width of the folders panel smaller to a certain point, then BANG, it's gone. The same goes for resizing it from nothing to something. If you only drag the mouse a small distance, the folder frame will not resize. If you drag it to a certain width, then the folder pane will pop back up.