seems to be a PSS issue, the WINDOWSIZE is not taken in consideration if another command is joined in the same button.
you can fix the pb by doubling the button like this:
// GLOBAL
$init_ps_global(panel.switch,0)
// SHOWPANEL
$showpanel_c(panel1,$ifequal(%panel.switch%,1,1,0))
$showpanel_c(panel2,$ifequal(%panel.switch%,2,1,0))
// BUTTONS
$ifequal(%panel.switch%,1,
$textbutton(10,5,50,12,PANEL1,PANEL1,SETGLOBAL:panel.switch:0;REFRESH,fontcolor:191-228-255,fontcolor:255-0-0)
$textbutton(10,5,50,12,PANEL1,PANEL1,WINDOWSIZE:384:236;REFRESH,fontcolor:191-228-255,fontcolor:255-0-0)
,
$textbutton(10,5,50,12,PANEL1,PANEL1,SETGLOBAL:panel.switch:1;REFRESH,fontcolor:63-100-127,fontcolor:191-228-255)
$textbutton(10,5,50,12,PANEL1,PANEL1,WINDOWSIZE:384:576;REFRESH,fontcolor:63-100-127,fontcolor:191-228-255)
)
$ifequal(%panel.switch%,2,
$textbutton(80,5,50,12,PANEL2,PANEL2,SETGLOBAL:panel.switch:0;REFRESH,fontcolor:191-228-255,fontcolor:255-0-0)
$textbutton(80,5,50,12,PANEL2,PANEL2,WINDOWSIZE:384:236;REFRESH,fontcolor:191-228-255,fontcolor:255-0-0)
,
$textbutton(80,5,50,12,PANEL2,PANEL2,SETGLOBAL:panel.switch:2;REFRESH,fontcolor:63-100-127,fontcolor:191-228-255)
$textbutton(80,5,50,12,PANEL2,PANEL2,WINDOWSIZE:384:576;REFRESH,fontcolor:63-100-127,fontcolor:191-228-255)
)