When there are enough tabs open so the Sidewise window has a vertical scroll bar, clicking on the X to close tabs doesn't work a lot of the time. This is because the (invisible, unless you're scrolling) scroll bar is on top of the rows, and takes over the very right edge where the close buttons are. To actually close a tab you need to hit the very left edge of the button, which is difficult.
Not sure what the solution is, but adding a gap on the right would be a very helpful workaround.
Customer support service by UserEcho