Qt: Can mouse event handlers block one another?

Question

I have a simple parent widget that reimplements mousePressEvent/mouseReleaseEvent. The parent's child widgets use enterEvent/leaveEvent. When I hover the mouse over the child widgets, leaveEvent/enterEvent executes, but when I click and hold the mouse, mousePressEvent executes, but enterEvent/leaveEvent goes silent (in other words, no click and drag). Any ideas about what could be causing this?

Answer 1

If you press and hold down the mouse button on a widget then that widget grabs mouse events until you release the button. This is not a special feature of Qt, you can find similar behaviour in every other GUI APIs I know.

Take a look at the relevant part of the Qt documentation:

QWidget / Events:

mousePressEvent() is called when a mouse button is pressed while the mouse cursor is inside the widget, or when the widget has grabbed the mouse using grabMouse(). Pressing the mouse without releasing it is effectively the same as calling grabMouse().

void QWidget::grabMouse ():

Grabs the mouse input. This widget receives all mouse events until releaseMouse() is called; other widgets get no mouse events at all.