add - means just that - AT THE END
Add means add. Doesn't really specify where. You can add to list, map, set, dictionary or sorted collection and it will have different placement in each. If you add to sorted collection it won't add at the end but at the place its sorting order indicates. For unsorted collections it just adds at the end because it happens to be the easiest and most performant operation. English word "add" doesn't have placement connotations, just expanding some collection to include given element.
append - is more precise , again at the end
Yes, but append is not used here for exactly that reason - it suggests position, which you can't guarantee if sorting is enabled. What should append b do to a sorted collection like this a,d,g? a,b,d,g? It breaks the meaning of append. a,d,g,b? It breaks sorting order. Both are wrong answers so if something can be sorted append operation is not included. On unsorted collections like QList or QString append is included because its meaning is clear and doesn't break container's assumptions.
insert - means at selected place
Agreed and it does just that. It could be more clear as insertAt, but the parameter clearly suggests that anyway.
for example suggested "is sorting enabled " is NOT a feature of TextEdit.
I mentioned it in context of QListWidget, which is a collection (list) of entries so it's obvious that it would sort those entries.
It's not clear what would it sort in a QTextEdit - letters, words, sentences, paragraphs, text blocks, lines, something else? What if text has markup? What if there's block formatting? Should it break that, split blocks or reinterpret before sorting? Not clear at all. Different people different needs, so again - it doesn't try to guess for everyone.
append - does not automatically advance line / index
Why would it? Advancing a line would mean adding data to the collection that you didn't put there yourself. What if you wanted to append contents of a streaming text buffer (eg. from a real time chat) and it kept inserting line breaks whenever new data chunk arrived? It would totally destroy the data and be kinda excruciating to workaround (inserting artificial backspaces?). If you want a line break just append a line break. It's up to you as a user to implement behavior you need.
