An A320 was taken off a SID but when re-cleared to a fix on that SID; the FMGC transitioned to a SID point downstream just as the DIRECT button was pushed causing a track deviation. The crew did not expect the FMGC transition which was different behavior after installation of new FMGC software.

Narrative

Taken off SID by Controller then assigned heading 190 to rejoin the Offshore Departure SID. Subsequently cleared Direct to SEGUL intersection. I commenced left turn while clearing visually left for traffic. First Officer went heads-down to enter the direct routing into the FMGC via the MCDU. As he initiated the direct; I scanned my ND display to get an approximation of my roll out heading. In the new FMGC software in the Airbus; the new fix is not displayed in the upper right corner until after the command is executed. As the First Officer executed the direct command; and I continued my left turn to align my heading with the ND displayed track; I glanced in the upper left corner to see that the FMGC was actually giving us guidance directly to the MCKEY intersection. I announced our error and immediately started a turn back to the right to correct our course back to SEGUL; as the First Officer reentered a new direct routing command to SEGUL in the MCDU. It was about that time that the Controller queried our heading and we verified that we were cleared to SEGUL. In our post-error discussion; the First Officer said that he thought the execution error happened as he inserted the direct routing as the MCDU was 'auto-sequencing' and rearranging pseudo waypoints; etc. The Controller made no further comments over the frequency regarding our brief overshoot turn beyond the direct SEGUL routing. The overshoot was approximately 30 degrees beyond the heading we should have initially rolled out on for the direct routing to SEGUL. We were on the erred heading 15 seconds or less. We corrected our error promptly. To our knowledge; there were no other aircraft in our immediate vicinity according to information displayed by TCAS; and no loss of separation occurred. However; this minor deviation did uncover and highlight for me; the potential with the new Airbus FMGC software; to be led toward a navigation waypoint before you can actually verify it on the ND; as is built into Airbus pilots' previous habit patterns.