Currently the positionAt() method uses a 'QTextLine::CursorBetweenCharacters' parameter to get the character position at the given 'X' position. This is useful if you want to place the cursor, but for manipulating characters, it is less so and even gives odd results in various situations.
Please consider adding a CursorPosition parameter that defaults to 'QTextLine::CursorBetweenCharacters', but which can be specified as 'QTextLine::CursorOnCharacter' when needed, in a similar way to QTextLine::xToCursor() which it eventually calls.