I have a sort of "child plugin" within my theme, but I'm not sure if any of the changes took effect. In any case, changing max-width to width there had no apparent effect.
Erasing the width setting in the dashboard erased the slide. Setting it to the full width of the content area did not help. This figure defines the area that the element takes up within the content area, and should have no effect on its ability to float horizontally within that area, nor to admit text alongside it without forcing a line break.
The problem is definitely that somehow a line break is being generated, as can be seen from the text being in a separate div.