Insert an expanding separator after the window buttons plugin. If a separator already exists, set it to expanding. To change this property, right-click the panel and select Panel->Panel Preferences. On the Items tab, either select the separator item or add a new separator item. Click on the Edit button and check the Expanding checkgox.

Cool!
It's solved.
thank you1

In xf ce4.8, my panel occupies the whole upper border in the screen where I have app menu in the left and icon's area, clock and action buttons in the right. In the middle of the panel, there is a long area of "windows button"
after updating to 4.10, the length of "windows button" change according to the opened apps, so that icons area, clock and actions button are no longer fixed in the right side. I am not familiar with the clock and volume mixer appearing in the middle nor in the left.

So what should I do to make "windows button" length fixed?

Thank you.