I wish I could cancel that note entry quickly with the Esc key in Windows.  

Option:  a hot key that removes the "extra" amount of rhythm from the first note in the measure instead.

(example:  in 4/4, I mistakenly put dotted half, quarter, quarter) - hotkey removes the dot and fixes it from the beginning.   Common type of entry error, I think.

I would also like to see this dialog updated...

Some thoughts:
1. Could the default action item be user defined in a preference pane?
        - rarely would I default to "Leave the measure alone" - and clicking a different choice pulls hands away from both keyboards
          to the mouse...
        - 99.99% of the time this dialog pops up is due to speedy/simple entry errors
          by the user... fixing the error with a "cancel" or "undo" would speed things up.

2. Could a "delete last note entry" be added as a choice?
        - This would quickly undo the rhythmic issue and allow the user to continue entering notes immediately.

3. Could the highlighted item be toggled via the arrow keys? This would eliminate moving again to the mouse to "click" on a different selection.

Thanks!!

Don't like the mouse? On my PC, TAB moves focus from section to section in the window. You can use arrow keys to cycle through the choices, or type l, d, m, or k to choose the statement, then tab to OK. Does the Mac not do this?

… On my PC, TAB moves focus from section to section in the window. You can use arrow keys to cycle through the choices, or type l, d, m, or k to choose the statement, then tab to OK. Does the Mac not do this? …

Yes, and No, Mac OS works differently.

Indeed you can customize Mac OS’ keyboard shortcuts.

This is done in the control panel Keyboard.

You can make it so that the Tab key moves focus in text fields and lists, or moves focus between all control options.

Also (for disabled users who can not use a mouse) it is (not very fast, but) possible to use the keyboard to

- highlight the menu bar,

- cycle through the menu titles in the menu bar,

- when a menu title is selected, pull down that menu,

- when a menu is pulled down, cycle through the menu items,

&c. &c.

Yes! Even if you quit the window-- which you can ONLY do by selecting one of the undesired options-- you're forced to do several undos.