Hi Lucuano,
It could be one of two things.
1st a Handler issue. This is why Cris recommended to apply the note
2166234 - SAP GUI for HTML: Dynpro too small on start -> parameter ~set_sizing_timeout
2nd it could be a "Compatability Mode" issue if it is occurring via IE when using two different Web Technologies in combinaion
However usually would get a popup warning if this was the case.
See the notes
1924378 SAP GUI for HTML: Info about standards and quirks rendering modes
Regards,
Oisin