second, I found a bug. In my opened pdfs there are some pdf's titles which are started with Chinese words, and I found if the Tab list becomes longer than the screen, those pdf's titles are gone from the Tab list. But they are still in the window list.
for example:
The red box is a pdf with chinese character in the window list

however you can't find it in the Tab list

PS. my pdf xchange viewer is version 2.5.316