How to lock Column widths in Mac mail

In the last couple of weeks, I've noticed that the width of my columns in email are not "holding." I'm using Column Layout, and have the sidebar and preview on the left side of my screen. I have a preferred width that I'd like them to be, but recently they keep on "shrinking" back to what appears to be a pre-set size (that I don't like or want). How do I "lock" the columns? Stretching them doesn't "hold" them. Thank you!

[Re-Titled by Moderator]