ControlDragging was: Re: [lazarus] CVS Changes

[Robert Scott Horning]

Marc Weustink wrote:

> Mouse.Cursorpos is given in screen coordinates, not in form coordinates.
> Onmousedown you store Cursorpos (or better, the given X and Y),
> onmouseUp you calc the difference with the new Cursorpos (or better, the
> given X and Y) THe difference is added to the Left and Top.
>
> I committed a solution which normally should work, however it doesn't due
> to a bug in the MouseMove and MouseUp handlers
>
> MouseMove / MouseUp should always give the mouseposition X and Y in the
> Sender coordinates.  It does as long as the up event occoured in the
> control itself. However if you are outside the control you get the X and Y
> in Form coordinates.
>
> Marc

Having done quite a bit of Windows programming, especially with the Mouse
Move/Mouse Up events, this is not correct.  The coordinates should ALWAYS be in
relation to Sender (even if the X & Y are negative... which is possible even for
Form or even Screen coordinates.)  The problem comes when you try to move
something, and as soon as you cross a threshold (like the edge of a control) it
jumps suddenly to a totally different location on the monitor.  Having made
mistakes like this in some of my GUI programming has caused hours of grief when
I got the coordinate systems mixed up.

--
Robert Scott Horning
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