CaretAffinity
Visual side of the caret at an ambiguous boundary — either a soft-wrap line break (right edge of the upstream line vs left edge of the downstream line) or a BiDi run boundary (LTR-end vs RTL-start). The same hint resolves both cases (Mozilla unified hint pattern: see https://bugzilla.mozilla.org/show_bug.cgi?id=1213589). Without an explicit affinity the caret position at such boundaries is undefined: codepoint indices alone cannot disambiguate "end of line N" from "start of line N+1" when both map to the same buffer offset.
public static readonly CaretAffinity Downstream = 0Caret renders at the left edge of the downstream side — start of line N+1 at a soft wrap, or start of an RTL run at a BiDi boundary. Default when nothing else dictates: forward typing, click landing inside a glyph, programmatic SetCaret without a specified affinity. [[]] of this enum and of any enclosing TextSelection.
public static readonly CaretAffinity Upstream = 1Caret renders at the right edge of the upstream side — end of line N at a soft wrap, or end of an LTR run at a BiDi boundary. Set explicitly by End / Down / any movement that lands the caret at the close of its visual line.