Solution : https://service.sap.com/sap/support/notes/868919 (SAP Service marketplace login required)
Key words : 
textedit control shows incorrect behaviour, input method editor, textedit control, readonly mode, change text, gecko browsers, readonly attribut, html element, folling patches, locating anddownloading
Related Notes : 
       
| 768063 | ITS TextEdit: Cursor cannot be set in read-only mode | 
| 563161 | SAP GUI, SAP ITS & SAP IGS patches on Service Marketplace | 
| 335521 | ITS patch installation of kernel files | 
| 330793 | Downloading patches |