Text transform

CSS's text-transform: the element draws different characters from the ones it was given.

TextTransform is the resolved value; TransformedText is the drawn string and the map back to what the author wrote, which is the part of this feature worth reading about.

Saying how a word is presented without changing what it says. A heading in small capitals is a fact about the theme; the label is still the label, and a search, a screen reader, a copy of the text and — the one that bites — an editable field's caret all have to go on seeing what was typed.

That last one is why this arrived later than the three-line keyword table it looks like.

⚠ A full Unicode case mapping changes the string's length. straße uppercases to STRASSE: six characters in, seven out. fine becomes FINE. From that point on, an index into what was written and an index into what is drawn are different numbers — and TextRun.Start, TextLine.Start, every caret offset and TextField's selection are all indices.

Shipping the four keywords without a map between the two puts the caret in the wrong character of an editable field, silently, and only on the strings that happen to expand. That is a worse outcome than not having the feature, which is why it waited.

⚠ And .NET's own casing would never have shown the problem. string.ToUpperInvariant, ToUpper in every culture, and Rune.ToUpperInvariant over all 1 112 064 scalars implement Unicode's simple case mappings, which are one code point to one by definition — measured over the whole code space, not assumed. So an implementation written on the framework's casing is perfectly caret-safe and draws STRAßE, where every browser draws STRASSE. A different defect, and a visible one instead of a silent one.

Both halves are closed together. SpecialCasingTable is the Unicode Character Database's unconditional SpecialCasing.txt rows, generated beside the segmentation tables by Tools/Vixen.UnicodeTableGen; TransformedText applies them and records where every index went.

Four classes, one property.

The property inherits, which is what makes it work on the text child a markup interpolation emits rather than only where it is written — the same reason text-decoration-line inherits here and does not in CSS.

The map, for a caller that needs to cross between the two strings:

⚠ The two conversions are not inverses, and that is the behaviour a text field wants. Source to drawn and back is the identity; drawn to source and back is not, because both units of the SS map to the one ß the author typed. There is no caret position between them, so a click in the middle of an expansion snaps to its start.

Identity costs nothing. No transform, or a transform under which every character kept its length — hello to HELLO — allocates no arrays and hands back the same string instance, so the shaping cache's fast path and the element's own block cache go on meaning what they meant.

UiDocument.TextTransformOf is read by UiElement.Block, before anything is shaped, wrapped or measured. A capital is wider than its lowercase in every text face, so a transform applied at paint would draw a paragraph the layout measured at a different width and wrapped at different characters.

TextLine carries the map, which is what keeps the distinction from leaking:

• TextLine.Start, Length, CaretOffset and CaretPositionAt speak the element's own string.
• TextRun.Start and TextRun.Shaped.Text speak the drawn string.

A consumer that goes through the line needs to know none of this. One that reaches past it into a run is the single place the two spaces have to be held apart.

capitalize leaves the rest of the word alone. iPhone becomes IPhone, not Iphone. That is what the specification says and what browsers do, and it surprises people every time.

The first letter, not the first character. "hello capitalises the h and not the quotation mark. Word boundaries come from UAX#29 rather than from "the character after a space", so don't is one word and gets one capital.

A digit is not a letter unit, so 1st becomes 1St. Also the specification's reading, also odd.

Titlecase is a third case. dž capitalises to Dž and uppercases to DŽ.

The conditional case mappings. SpecialCasing.txt's remaining rows depend on surrounding context or on a language — final sigma, the Turkish dotless i, the Lithuanian retained dot. TextShaper leaves HarfBuzz's language unset on purpose so that shaping does not depend on the machine's locale, and the same reasoning applies here: a case mapping that changed with the operating system's region would make a golden image machine-dependent.

full-width and full-size-kana. Compatibility mappings for Japanese input methods, with no utility class in any framework this project follows.

A column header in the editor's own idiom:

Undoing an inherited transform on one child, which is what normal-case is for:

• Text decoration — the other typographic layer over a run of text.
• Text input and the input method — the control whose caret this had to keep honest.