I still have to look at the PM screen.
and for those seeking to fix this:
sendprivate.tpl
change line
<input type="button" name="preview" value="{$lang.MESS_PREVIEW}" class="form-button" onclick="doPreview();" />
into:
<input tabindex="4" type="button" name="preview" value="{$lang.MESS_PREVIEW}" class="form-button" onclick="doPreview();" />
and in mess_handler.tpl
change the following 2 lines:
<input type="submit" name="buttsubmit" value="{$lang.BUTT_SUBMIT}" class="form-button" />
<input type="button" name="preview" value="{$lang.PREVIEW}" class="form-button" onclick="doPreview();" />
into:
<input tabindex="3" type="submit" name="buttsubmit" value="{$lang.BUTT_SUBMIT}" class="form-button" />
<input tabindex="4" type="button" name="preview" value="{$lang.PREVIEW}" class="form-button" onclick="doPreview();" />
Not much of a difference with the original code, it just adds a "tabindex="?"" to every line.