It's not possible to animate the indicator or expand/collapse in the new QML TreeView.

If you try to put an animated Behavior on the rotation of the indicator arrow, it seems to work at first but then the indicators of ancestor items will spuriously rotate when you open/close descendants.

The example in the docs for a supposed animated indicator does not animate, at least in 6.5. If it only works in some later version, that should be indicated in the docs. (I'll try again when we finally update to 6.8 LTS). But the fact that none of the styles in the baseline code on GitHub have animated indicators points to it not working in any version.

I realize that the way TreeView is implemented as a TableView makes it harder to animate expand/collapse, but this really needs to be done somehow.

Of all the possible UI animations, animating a tree open/closed is the most justifiable/needed, so the user's eye can track what's happening, instead of the transition just being a jarring pop.

It's kind of wild it was released like this and has never been improved since. It makes UI created with it look like it's from 1997.