Support setting tabs width

[benoitkugler]

I would like to support setting the width of tabs in a run, following the CSS 'tab-size' property.

That could be exposed with the method

// SetTabsWidth alters the run, replacing the width of every tab character (U+0009).
//
// [text] is the input slice used to create the run.
func (*Output) SetTabsWidth(text []rune, spacing fixed.Int26_6)

(At a first glance, the implementation would be somewhat similar to Output.AddWordSpacing)

What do you think ?

[andydotxyz]

A great idea, however I don't think it is quite that straight forward - I think you also need to have a starting position, because tab alignment is actually relative to the page and not to the first character of a run...

[benoitkugler]

Well, I think what I've in mind is indeed a lot simpler than tabs alignement. I think what you describe is something about "dynamic" adjustment, depending on the position of the run with respect to the page start ?

[andydotxyz]

What you outline I think only works when the tabs are right at the beginning of a line. But for example "\t\tpoint a:\t detail here" won't work as expected.

A while back I hoped to contribute the algorithm we use in Fyne for this work but licensing doesn't allow.

[hajimehoshi]

Tabs work in much more complicated way in e.g. MS Word's tab stops. Are tabs really this library's scope?

[benoitkugler]

What you outline I think only works when the tabs are right at the beginning of a line. But for example "\t\tpoint a:\t detail here" won't work as expected.

A while back I hoped to contribute the algorithm we use in Fyne for this work but licensing doesn't allow.

Aah ! Indeed, you are right. Browser rendering for the string
\t\tpoint a:\tdetail here
with tab-size: 10 (meaning a tab is 10 spaces) :

The expected result is to align the content after a tab to a "tab grid" (displayed in red in the example).

Tabs work in much more complicated way in e.g. MS Word's tab stops. Are tabs really this library's scope?

Well, since it seems not trivial, I could be great to provide it for all go-text consumers right ? (If we find an API that suits everybody.)

[hajimehoshi]

I'm not sure it is really possible to provide a generic model for various use cases including MS Word, but I keep my fingers crossed :)

[andydotxyz]

They way we do it in Fyne is to pass in the "current offset" into the tabletop function so that they incremented by "offset mod tabwidth" and after that it's just the basic math.

OK not quite that simple - you might need 2 offsets: the current offset and the offset used on subsequent lines.