I think the tab order isn't set on the full reply page
That was it...
in newreply.tpl change
<input type="button" name="preview" value="{$lang.DO_PREVIEW}" class="form-button" onclick="doPreview();" />
To:
<input type="button" tabindex="4" name="preview" value="{$lang.DO_PREVIEW}" class="form-button" onclick="doPreview();" />
I still have to look at the PM screen.