I think this because DoM is rendering all the space inside the <Text> tag literally - perhaps it should collapse it, but I can't find a reference for that...

I found this in the 2014 subtitle standard:

"All space characters present inside the content of a Text element shall be rendered, even when located
in a Font, HGroup or Rotate child element – meaning that consecutive space characters shall not be
collapsed. Space character(s) located outside of a Text element shall not be rendered. Also, as specified
in Section 5.11, control codes (e.g., carriage return, line feed, tab, etc...) shall not be rendered."

I moved the "side issue" into 0002557 as it's easy to miss stuff like that.

Confirmed, that does seem to be what it is.

The subtitles XML in my example puts Font elements for italics on a separate line within the Text element, with space for indentation.

The subtitle before the one pictured is not italicised and so does not have any excess space within the Text element, and it does render centred.

But...

There's also excess space after the closing tag of the Font element, before the closing Text tag. That's 4 spaces less than the space at the start of the Text element, due to the indentation pattern. But it looks like the subs are off-centre by more than 4 spaces.

So I am wondering if DCP-o-matic is correctly taking into account excess space at the start of Text elements, but ignoring at the end.

This DCP was made with easyDCP Creator+ 3.7.6. So, even if the real fault is with the DCP, probably there's lots more like this out there.

Sorry, I would include XML extracts which would make this easier to describe, but I know that Mantis renders them incorrectly.

It's a good point. It looks like something weird is going on with subtitle spacing when there is a combination of newlines and spaces within the text.

My suspicion is that it's ignoring spaces which are "outside" of a line break.

i.e. \n subtitle text \n is treated as [ignore]\n[include]subtitle text[include]\n[ignore]

In the case above, the problem is that the excess space is after the line breaks on both sides, so only the spacing before the subtitle text is included and it shifts the subtitle right.

It may be that DOM's interpretation of spacing is perfectly correct and the fault is with the original DCP, in which case this should likely be closed without any action. However, if the space "outside" the line breaks should be included according to the spec, that'd make the subs render (almost) centred in this case.

Mantis removed some of the spaces from my example above. Let me try again:

    \n    subtitle text    \n    

[ignore]\n[include]subtitle text[include]\n[ignore]

I think you're right, there's more to do here...