Translating Mouse Hit 'x' Offset to Caret Position

Conventionally, caret position (cp) may be selected by clicking either the trailing half of character "cp-1" or the leading half of character "cp". This may be implemented as follows:

int iCharPos;
int iCaretPos
int fTrailing;
ScriptXtoCP(iMouseX, ..., &iCharPos, &fTrailing);
iCaretPos = iCharPos + fTrailing;

For scripts that snap the caret to cluster boundaries, ScriptXtoCP returns ftrailing set to either 0 or the width of the cluster in code points.