Editor >> Pasting code does not conform into proper indentation format

If pasting code into the editor, since the editor actually uses '\t' characters instead of standard 4 spaces for indentation, the indentation will most likely not match the overall indentation of the document.

The reason to keep '\t' characters as default indentation in the editor is because that is the default of PyQt / Qt's QTextEdit. Subclassing QTextEdit entirely and writing indentation from scratch would be the best fix for this.

Start by examining the editor's PyQt/Qt QTextEdit behavior for indentation and pasted text. Confirm how tabs are currently used and define the expected indentation after pasting code; done means pasted code conforms to the surrounding document's indentation without disrupting existing text.

索引モデルが issue の本文から書いたものです。