Rare problem that happened by accident. Accidentally clicked the tab of a resource and dragged it slightly to the right before releasing it. It moved the resource tab one to the right but then left the resource window empty. I did it again on purpose and had the same result. In this case I clicked the first tab, drug it slightly to the right, and released. I haven't yet succeeded in getting it to do it a third time. It's easily fixed by clicking a different tab and clicking back. The glitch is shown in the bottom right panel of the screenshot below.
