Weird thing was, when you clicked into a post, the sidebar was fine.
What I know about HTML could fit in a sock, and the thing that sock-full of knowledge would be most useful for is beating my computer. But I don't give up easily, so I checked the coding and tweaked and prodded and finally, cursed at my computer and spent twenty minutes throwing beans at it.
Later, I checked the Blogger help pages. I did everything they said to do, and nothing happened. I realised it was probably a Page Break issue, but couldn't fix it even with the HTML fixes on the blogger help pages.
HERE'S HOW I FOUND THE PROBLEM:
