I created a new horizontal tab group and then started right-clicking on tabs in the top group to move to the group below. If I remember right, I moved a .c file below and then when I clicked on the .h file to move it below, it disappeared. Then it seems now that every time I try to open a corresponding (i.e. same base name) .c or .h file, if I click on the tab for the other one it disappears. This is very broken!