LeftMouseButtonDown + Up in a window

[Tomio]

I know there are many topics concerning the question: How to catch LeftMouseButtonDown + Up in a window?

On Windows I prefer the #WM_LBUTTONUP/DOWN solution.
But the program should run on Mac + linux too and I can't find a solution for this.

I would like to understand, why there still is no native PB command to catch LeftMouseButtonDown/Up in a window.
The task looks simple (for me as a layman). But there seems to exist a big barrier.
Is an expert out there who can explain this?

tomio

[Tomio]

I forgot, in case someone is interested:

My window has a "self-built" header with the usual buttons (exit, maximize,...) plus some more.
The window contains a movie and I don't want a menu bar.

I need a LeftMouseButtonDown/Up somewhere in the header to move the window.

My solution for Windows+Mac+linux (I hope) in the end: an additional button with toggle-flag.
As long as toggle-state = 1: the window moves with the mouse.

tomio