Writing Causes Page to Jump
most likely, one of two things is happening:

when you are typing you are accidentally touching the trackpad and moving or scrolling or clicking outside the text box you are typing in.

or when you are typing the case is flexing causing accidental clicks (heavy keyboard typists beware)

The clue here is the backspace functionality of the browser, as when inside a text entry field, the backspace works as a backspace delete, but when outside of a text entry, pressing backspace works the same as pressing the back button in the browser (i.e. go back to the previous page)