Canvas of TForm not working on Linux

[Koval]

I have a simple code on ButtonClick event

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form1.Canvas.Brush.Color:=clBlack;
form1.Canvas.FillRect(Rect(0,0,100,100));

But This Code do nothing;

When I try paste this code in OnPaint event, all works good;

I have trial CrossVCL and Linux MInt 19.3 Chinamon desktop.

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[Eugene Kryukov]

On mac & linux drawing to Form or Control canvas outside paint event impossible because of system limitation.