More on a recent thread on this:

Problem: sometimes the background of a container or shape will show through in non-selected textboxes, making it appear as though a lot of textboxes are all selected & highlighted at once. Even typing in one of these "highlighted" textboxes does not remove the highlight, which appears to actually be the backcolor of the container/shape showing through.

This occurs even when shape/container is definitely "underneath" and has Backstyle set as transparent.

Solution: best I've found so far is to set the container/shape backcolor to the backcolor of the textboxes, in this case white. Seems odd to be setting backcolor of transparent object, but does seem to work...