esmi,
Thanks for the quick reply. I figured out what was going on, and I’ll explain in case this happens to others. I had wanted to put a vertical line between my content area and the left sidebar, but I had noticed that the line disappeared until I went far down enough that the sidebar content had ended. In other words, the sidebar was overlapping the vertical line I had drawn. Being a beginner, the best remedy I thought of was to create a large left margin in my content area; a margin of ~300px did the trick, and the line reappeared.
Well, I later forgot I had done that, and it was what was causing all the content of my posts to jump to the right in Visual Editor. Sure enough, when I undid the margin-left command, Visual Editor went back to normal.
This left me with the original problem: the vertical line had once again disappeared. But I’m rapidly learning more css and html, and I figured out that instead of setting a content-area left margin, I could set the width of the left sidebar to be slightly smaller. (I’m using the Weaver theme; this command was “#primary.widget-area {width: 220px;}” in the Advanced Options section of the Weaver Admin editor.)
Voila! Now I’ve got a vertical line and the ability to edit normally. Thanks!