This was added via {T123153} in 2016. It does have so many issues:
* It's rare that a dialog title is so long that it appears cut-off. A solution that tries to improve such an edge-case should take care to not introduce new issues. But this one does.
* Users typically don't expect a tooltip on a window title. I believe it's fair to assume that the majority of users running into cut-off titles don't think of hovering them with the mouse.
* It doesn't help users on touch devices.
* In contrast to the minor benefit, the duplication is always a problem for e.g. screenreader users. The same text is always read twice.
* It's useless and confusing in all situations where the title fits. I believe it's fair to assume that this is the vast majority.
* Reading the short discussion in T123153 I do have the impression that:
* Even back in 2016 it was clear this is just a band-aid patch, and probably not even a good one. (Quote: "Let's do that as a start.")
* It's hard to tell what the reasoning for the tooltip solution was, other than being quick to implement. Nowadays the browser tooltip feature is considered to be inaccessible, see e.g. https://sarahmhigley.com/writing/tooltips-in-wcag-21/.
* The implementation is half-baked. The duplicate tooltip is only set under very specific circumstances. Only when the `….static.title` property is used. But:
* Not all dialogs do this. There are multiple ways to achieve the same, e.g. using a config option or calling `this.title.setLabel()`. Only some duplicate the title.
* Many dialogs change the title dynamically. I could not find a single one that makes sure the tooltip is updated accordingly (or removed). Instead the tooltip says something wrong then. This probably qualifies as a bug in the majority of situations.
* It's entirely undocumented, as far as I can see. Devs using a subclass of the Dialog class aren't aware of this and it's implications. This backs both the "band-aid" argument as well as the argument that this behavior potentially causes bugs in all code using #oojs.
At this point it's better to ignore and work around this behavior (by not using `….static.title` but manually calling `….setLabel()`) or revert [the patch from 2016](https://gerrit.wikimedia.org/r/297973).
A proper solution should act only when necessary, i.e. only when the title is actually cut-off. And even then a tooltip might not be the best solution. Instead:
* The font size could be lowered a bit.
* It could wrap to 2 lines.
* The title could start a scroll animation. (Not that I think a distracting animation is a good solution, but it's a solution I have seen in other places.)
* Another truncation algorithm can be used, e.g. one that puts the "…" in the middle instead of the end.
* The tooltip could be shown on click instead of on hover.
* …