Should we take the same SDL_GetModState() approach for the shift/alt keys too? Or do you think that's unnecessary.

As far as I can tell we don't use ALT at all but I guess we do use SHIFT and track it specifically unlike CTRL. I think it makes more sense to leave it how it is though because it would actually be slightly more work CPU wise to change it. Either we have a shift_down variable/expression and pass it to nk_input in the same place we have now (when the key event is a SHIFT) which makes the shift_down calculation completely redundant with down, or we remove SHIFT from the switch and call nk_input for shift above it which means many many unnecessary function calls.

That was/is one of the issues with GLFW backends. nk_input is really meant for events and so calling it every frame for a state is just inefficient even if it's sometimes functionally the same for the user. I was actually debating adding all of these to the key callback and guarding all of them for that reason even though the user can't tell the difference:

    nk_input_key(ctx, NK_KEY_TEXT_START, glfwGetKey(win, GLFW_KEY_HOME) == GLFW_PRESS);
    nk_input_key(ctx, NK_KEY_TEXT_END, glfwGetKey(win, GLFW_KEY_END) == GLFW_PRESS);
    nk_input_key(ctx, NK_KEY_SCROLL_START, glfwGetKey(win, GLFW_KEY_HOME) == GLFW_PRESS);
    nk_input_key(ctx, NK_KEY_SCROLL_END, glfwGetKey(win, GLFW_KEY_END) == GLFW_PRESS);
    nk_input_key(ctx, NK_KEY_SHIFT, glfwGetKey(win, GLFW_KEY_LEFT_SHIFT) == GLFW_PRESS||
                                    glfwGetKey(win, GLFW_KEY_RIGHT_SHIFT) == GLFW_PRESS);

and same with these but that'd be a bit more complicated

        nk_input_key(ctx, NK_KEY_COPY, 0);
        nk_input_key(ctx, NK_KEY_PASTE, 0);
        nk_input_key(ctx, NK_KEY_CUT, 0);

Speaking of though, I've been using SDL forever and never knew or forgot a long time ago about the mod member of the key event so thanks for pinging me so I would see that. In looking into that after I saw another minor improvement I should have been aware of in the enum SDL_Keymod:

KMOD_CTRL = KMOD_LCTRL | KMOD_RCTRL,

And similarly for all the other 2 sided modifiers. D`oh!

So that's a small improvement we can make to all 6 SDL backends, not sure it's even worth a separate pull request though. I can do it if you want though.