I’ve noticed that with sliding panes on the little x to close the pane is sometimes invisible.
It’s always invisible on whatever pane is active, and if more than one pane can fit on the screen, the x for those panes are also invisible.
It is there if you mouse over it, the “Close page” tooltip appears, but the x can’t be seen at all. Once a there are enough panes for them to stack, the x appears.
I’m not sure if this is a bug or if it’s deliberate, but a reader has emailed me saying it’s quite confusing and I noticed it myself a few days before.