Bug? Z-order issue with tButton
lazarus / 31-LCL

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IPguy · 2012-05-16 01:16

When I stack a tButton over a tLabel, the z-order has no effect.  I am
not able to get the label to show up on top of the button by any
combination of send to front / send to back /back one / forward one.

Is this a bug?  I did not see anything for this in the bug list.

Laz: 1.1-36568-2.6.1, win32-Vista

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felipemdc · 2012-05-16 05:44

It is not a bug. You need to separate LCL controls into 2 kinds of
controls:

*TWinControl descendents: Windowed controls, they have a window and
therefore can receive keyboard focus. Cannot have real transparency
but can be shaped using TWinControl.SetShape. TWinControl descendents
can be grouped in 2 groups:
1->TCustomControl descendents which are controls implemented and drawn
in the LCL. See http://wiki.lazarus.freepascal.org/Roadmap#Status_of_T
CustomControl_based_controls_on_each_widgetset
2->Native controls. Those are implemented using native APIs (except in
the LCL-CustomDrawn widgetset). See http://wiki.lazarus.freepascal.org
/Roadmap#Status_of_native_controls_on_each_widgetset for a list

*Other TControl descendents which don't descend from TWinControl:
Those are just painted into the canvas of their parent and are called
non-windowed controls. So they have no keyboard focus and should not
be utilized for action controls, like buttons, because they are not
accessible for the visually impaired. TLabel is the most often used
control in this class. TSpeedButton too.

See a list of non-windowed controls here: http://wiki.lazarus.freepasc
al.org/Roadmap#Status_of_TGraphicControl_based_controls_on_each_widget
set

Logically since TLabel is just painted in it's parent canvas that it
cannot be over a button.

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KpjComp · 2012-05-16 07:01

Like @felipemdc said, TButton is a TWinControl so you can't place
TGraphicControls on top.  You can place a TLabel inside a TWinControl,
eg.  TLabel inside a TPanel, but that's not the same thing.

A TSpeedButton is a TGraphicControl, so you can use TLabel that is
also TGraphicControl on top.  This works because both controls are
painting on there parent's canvas.