Chom reap sour, Norum
There are two ways of editing your post. See the attached screenshot - both possibilities shown in red.
When you click on the small editing symbol on the bottom of the post then you get only the small editing window as you have shown. (This is called the "quick edit" button, I think, because it goes quicker, without loading a new page, for some small and simple modifications.)
But on the top there is another editing symbol with the text "modify".
If you click the "modify" button on the top you will get the bigger editing window, the same as when first writing a new post, with all the buttons in the toolbar to enter the codes for formatting and so on.
It is a little bit confusing.