If you like to center your index page or any other page , thiswindow Center.action is the Action you need.

thiswindow Center.action will position the window correctly, due to a build-in browser detection script.

This you can use as a Head Action, mainly.

thiswindow Center.action can be used in conjunction with thiswindow Resize.action to insure your window is the size where you want it.

To insure the correct position at the center of the visitors screen. You must make sure the Scroll, Status, Resize etc. matches the intended "Browser Window Chrome" (?) settings of the window you want to render.
If you have set the current window to be without any "Browser Window Chrome" uncheck all check boxes. If you don't, the window will not position itself in center of the screen.

Notes:
thiswindow Center.action will not enable nor disable any of the "Browser Window Chrome" (?). These options are for window size calculation only.

Limitations:
thiswindow Center.action
Works with single HTML-pages only.
Not with framed files.