1.Please change IFDEF WINDOWS to IFDEF LCLWin32 since qt & gtk2 works on windows platform too.
There are three procedures where I had to put IFDEFs. I'm not sure that each should be changed to IFDEF LCLWin32 (maybe it should be IFDEF LCLGtk2 or something else). To do some testing I installed gtk2 on Windows to see the behavior in this combination. Uh... wasn't quite pleasent experience... seeing this face of our Lazarus.

The first IFDEF is in SetShowCheckBox procedure. It is there to avoid buggy behaviour of TCheckBox I experienced when testing on Linux. To see what I mean, I'm providing the application which explains and reproduces this (attached to this message) note that the modal form must be opened the second time to see the bug.
Now, In Windows with gtk2 test it did not reappear! Is it a Linux bug?
It should be tested on Linux + qt combination. The line CheckBox.Color := clBtnFace prevents the bug and does not spoil the check box's appearance. Thats why this IFDEF is there. Here, I'll leave IFNDEF WINDOWS for now.
The second IFDEF is in DateEditorKeyDown procedure. On gtk2 (doesn't matter Windows or Linux), the CheckBox.SetFocus method seems to automatically generates one click. On win ws it is not so. I'll put IFDEF LCLWin32. Can you test it on qt, please? Should it be IFNDEF LCLGtk?
The third IFDEF is in DropDownCalendarForm procedure.
If the control is on a form shown modally then the calendar form cannot get user interaction unless it is shown modally too. It is a gtk2 behaviour, regardless of OS. Should be tested on qt, I'll put IFNDEF LCLWin32 for now.
By the way, can you tell me where can I see the list of all possible IFDEF's?
