WS_EX_TRANSPARENT - What does it actually do?

Question

In my project, I create a form with the opacity controlled by the user. If the form was fully transparent the mouse events 'fell through' (without my intervention), otherwise my form handled them.

After reading this question and overriding the CreateParams property to set the WS_EX_TRANSPARENT flag on my form, it now allows mouse events to fall through when the opacity is any value <255.

This is exactly what I want, but it concerns me that I don't understand why it works.

From what I've read, WS_EX_TRANSPARENT is meant to make the form appear transparent by 'stealing bits' from the form below it in its Paint method, therefore:

1. The Paint method of my form and all the controls in it should never be called, right? As WS_EX_TRANSPARENT should cause Windows to override them, so why does it affect input handling but not the drawing of my form?

2. The opacity should have no impact on the handling of mouse events, as if Paint is being overridden the 'local' opacity should not matter, no?

Could someone explain, what this flag really does? How does it work?

Answer 1

WS_EX_TRANSPARENT makes your events (like mouse clicks) fall through your window, amongst other things. Opacity is a separate concept, it instructs window manager to apply alphablending when drawing your form. Those two things are not related, but when you combine them you get the effect you need in your case.

So:

1. Paint method of your form is called normally as it should, opacity has nothing to do with it.

2. Opacity does not have anything to do with mouse events, as described in the first paragraph.