Hello, Adam!
Your advice to try with fewer buttons was decisive. Thanks a lot.
The bug was found into the dialog constructor:
The last operator of the constructor was the call of some function. This 
function calls another one, wich calls anosher one, ... wich calls Invalidate 
for every button. But dialog constructor is not finished at this moment! No 
parent-child ralationship for dialog window interface elements are set!
Really childs mistake ...
Now I know much better what for we need SetupWindow function.
After replacing the function call from constructor to SetupWindow the desktop 
window is in peace and dialog window with buttons appears 10 times faster.
Alexey
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