获取Richedit的可见的起始行和最后可见行号

起始行：
用EM_GETFIRSTVISIBLELINE消息即可。
结束行：
Var
r: TRect;
lastvisibleline, lastlineindex: Integer;
Begin
richedit1.perform( EM_GETRECT, 0, longint( @r ));
r.left := r.left+1;
r.top := r.bottom - 2;
lastlineindex := richedit1.perform( EM_CHARFROMPOS, 0,integer(@r.topleft));
lastvisibleline := richedit1.perform( EM_EXLINEFROMCHAR, 0,lastlineindex);
end;
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