Use tabindex to navigate the invoice form only with the keyboard

Hello,

Previously (maybe before v0.4 I don't remember exactly) the create invoice button was conveniently located after the buttons allowing to select who was involved in the invoice. Now it's beside the title.

I find that the previous location was better because it allowed to navigate the invoice fields only with the keyboard. Now I fill everything without touching my mouse but to create the invoice I have to move my mouse cursor to the button in ordre to create the invoice.

I think that using something like the tabindex attribute would probably help without moving the button (because I assume that if it's there it is for a good reason 😄).

(BTW let me thank you again for this nice piece of software).