Testing on Safari 14 (macOS Big Sur).
In the Codespaces editor, I would expect cmd+right arrow to jump to end of the line - but this doesn’t work. cmd+left arrow, cmd+up, cmd+down (jump to end of document) are also not working.
Combining cmd+right with shift (which jumps to end of line and selects the text) is working. Another similar keybinding, option+right arrow works (to jump to the end of the next word).