If I set the window to 900 pixels wide and then display an issue, the window as a whole gets a horizontal scroll bar. But if I scroll down to the Activities section (and click on "All"), each individual activity has its own scroll bar.

This is Bad Design: In general, any scrolling area inside another scrolling area is hard to read and does not use the limited screen space well. In particular, the Activity items seem to be limited to about 3/4 of the available screen width, despite that the 1/4 of the width remaining on the right has nothing in it.