In Qt 5.12 the width of tool buttons with an icon, a menu and the style "icon alongside text" will be too narrow resulting in their labels being elided, e.g. "Add t…queue" instead of "Add to job queue". In Qt 5.11.2 the width was calculated correctly, and the resulting labels aren't elided. See the attached screenshots for examples.
This happens on both Linux and Windows, the platforms I can verify it.
Disabling the icon causes the width to be enough for the label not to become elided.
How to reproduce: I'll attach a short UI form. Open the form in a QtCreator compiled against Qt < 5.12 and it'll look OK (e.g. QtCreator 4.7.2 on Windows from the official installer). Open the file in QtCreator built with Qt 5.12 (e.g. QtCreator 4.8.0 on Windows from the official installer), and the labels become elided.