Don't shrink window if contents change

I’m writing an app that uses a Gtk3 window for a progress dialog, and it shows the current task being processed as the text of a progress bar. The problem is that, sometimes, the text is a little bit bigger than the available space, so the window grows to accommodate it (which is fine), but if the next text is shorter, the window shrinks again to the original size.

Is it possible to make the window “just grow, but not shrink”? This is: if the window has to be made bigger to fit a long text, allow it to grow, but if a new text is smaller, don’t shrink it.