Thanks for bringing this up, @Mhurd.
Pre-comment disclaimer: I don't have a tech background. That said, my suspicion is that this occurs in part because WordPress hardcodes the image width to 580px, at least until(?) the width is too narrow (by which time the right sidebar disappears entirely). It looks like this issue occurs in a strange middle ground, which the window is too narrow to fit both but is before the sidebar disappears, which happens when the screen is [??] px wide.
I can't replicate this issue on mobile, as the right sidebar is omitted, so this might be a rare edge case. Someone with experience in web design would have to confirm that.