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Layer's properties --> Fields tab, drag-and-drop editor layout activated
Trying to create a feature form, I'm not able to move fields to the form with drag-and-drop system. Actually, It worked at the beginning and once I added a category, it no longer worked. Dragging a field, it doesn't follow the pointer until landing in the right panel (see screenshot). So the only way to add fields is to use the right arrow button and the move it with other arrows.But That means there's no way to embed a relation form.

I cancelled the Layer's properties dialog and reopened it, began a new project, relaunched QGIS, all this without success.

Note that this also puts the pointer into a weird state where, unless hitting ESC, no dialog can interact with it.

Drag-and-drop function to build a form in 2.18 on Windows works smoothly.