GtkPaned gutter position changes when child resizes

[earlgrey <earlgrey free fr>]

 Hello,

I have 2 widgets packed in a GtkPaned :

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|         child 1                      |      child 2                 |
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When one of the childs resize itself ( ex : a GtkLabel whose displayed 
text lengthen ), the gutter position change.

How to avoid that properly ? Because the only solution I have found is 
to intercept the "size_request" signal  of the child and set the 
requisition->width to a constant !!! Its tricky ! like this :

void my_handler(
    GtkWidget        *widget,
    GtkRequisition  *requisition,
    gpointer        data)
{
    requisition->width  = 400; // could be anyelse constant number
    requisition->height  = -1;
}

Thanks