When resizing the tree view columns a fixed width value is set: https://searchfox.org/comm-central/rev/5391ea879779884999142a67920e99c83ea356bd/mail/base/content/widgets/tree-view.mjs#2083

This works okay as it doesn't allow to resize columns outside the limits of the table layout, but unfortunately it doesn't work as well as 102 used to.

The old XUL tree had a more "elastic" behavior in which a flex value was set on the columns in order to let them grow or shrink based on the size of the viewport or the application.
That allowed a more natural proportional sizing of the columns.

It would be nice to emulate that behavior and improve the experience of our tree view columns.

This is something that we will explore during this cycle to improve the general usability of the tree view, but it something that requires more thinking and prototyping so for now we're going to put it in the backlog.