Confusion with parameters of ClientToXY()

[Stoll_Meik]

I have to determine the exact coordinates of a mousedown-event on a canvas of a TWebPaintBox. For that I found the procedure ClientToXY which is defined as
procedure TControl.ClientToXY(AClientX, AClientY: single; var X,Y: single);

But when calling with single parameters, the compiler gives the message
Incompatible types: got "Double" expected "Longint"

When I change all 4 parameters to Longint, I get
Incompatible type arg no. 3: Got "Longint", expected "Double". Var param must match exactly.

When I then change par. 3 and 4 to Double, I get
Incompatible types: got "Double" expected "Longint"

There is only one file which has the declaration:
c:\Users\meik\Documents\tmssoftware\TMS WEB Core RSXE12\Core Source\WEBLib.Controls.pas

So I am confused and can't see what I am doing wrong. Delphi 10.3.3, TMS Web Core 1.6.0.0
Any ideas?

[Stoll_Meik]

...finally, I could solve the confusion. The compiler stopped at the line with the call to ClientToXY, but in fact the problem was in the sourcecode line below...