When you click and hold on a GtkExpander's label or disclosure triangle, then drag the mouse elsewhere and release, the expander still expands or unexpands (exactly as if you had simply clicked on the label.)

This behavior is different from GtkButton's behavior and the behavior of expanders in other UIs; if you click and hold on a GtkButton, then drag the mouse off the button, and release, then the button is not clicked.

To reproduce:

- Run gtk3-demo
- Run the "Expander" demo
- Position the mouse on "Details", click and hold the left mouse button
- Move the mouse elsewhere (e.g. off the GtkExpander window) so that the prelight state of "Details" disappears
- Release the mouse button
- The string "Details can be shown or hidden" is revealed

Doing the same thing on the "Close" button in the same demo shows the expected behavior.