I upgraded to Version 15.0.0.2042 Beta (64 bit) this morning and seems the window list tab-order is not kept between sessions now.

Also immediately after the update I suddenly got a tabbed interface, although I never use this. And when I opened a new file from the files tab it opened in normal mode, although all other open files were maximized. I could not reproduce this after restarting PL/SQL Developer.

Code assist is rather slow and so is switching between tabs (in the window list).

We'll check it out.

I had noticed that the window order within a workset seems to change when it is reloaded after the program has ended. Is the order dependent on last use or the order of arrangement at the time of termination?

The order should be the same as when you terminated. We are looking into this.