ない in つまらない and similar adjectives and their conjugation with すぎる?

Question

The usual path for conjugation would be: 1. 高い　→　高すぎる　 (drop い, append すぎる) 2. ない　→　なさすぎる (drop い, append さ, append すぎる) Is ない in つまらない treated as the negative ない which conjugates as 2.? Or is つまらない a single piece that conjugates as 1. ? Does つまらない conjugate to つまらなすぎる or つまらなさすぎる? Are both acceptable? If so, is there a preference for one form or the other? Do the conclusions that apply for つまらない also apply to similar adjectives ending in ない such as: - 危ない (ない explicit as おくりがな) - 汚い (ない hidden inside the word)

Answer 1

The ending い of い-adjectives normally becomes さ when you conjugate it with すぎる or そう. In the case of an adjective ending in ない, you should change the い to さ. つまらない→つまらなさすぎる

If you have a negative verb, you don’t change the い in the ない. For example: わらう→わらわなすぎる

That is unless your verb is only two characters in length to begin with, such as ねる,みる,する, etc. Those will have the final い become さ. みる→みなさすぎる