Lita, I tried the patch.  From the perspective of an OS X user, while I might expect that using the zoom gesture on a mousepad or using a mousewheel (the equivalent) to increase or decrease the font size, I would even more expect scrolling to work especially if scrollbars are present.  Clearly, scrolling is more important so, if it is not possible to bind Tk mousewheel events without affecting scrolling, I would abandon the mousewheel.  On OS X, the standard way to provide size adjustment (of fonts or images) is to provide "Bigger" or "Smaller" menu items with the standard keyboard shortcuts of Command-Shift-Equal (and Command-Equal) which is displayed as "Command +" (so the user on a US keyboard just presses the Command key and the =/+ key) and Command-Hyphen ("Command -").  The Apple OS X Human Interface Guidelines go into more detail and you can see these shortcuts in action in many standard OS X applications (TextEdit, Mail, Safari, etc).  As it stands today, turtledemo does not use the standard OS X menu bar where these commands would normally be placed.  And that's a bit of a separate problem because since turtledemo doesn't change the root menu it defaults to a Tk-provided one which includes things "Run Widget Demo" under the file menu.  To be a proper OS X app, turtledemo should customize the menu, at least removing the widget demo item and then it could add the Bigger and Smaller menu items to a Format menu.  Actually, the turtledemo Examples and Help pulldown options would ideally also be available in the standard menu hierarchy.  I'm not suggesting that is a requirement but that's what I think an OS X user would expect and what the Apple HIG would require.

https://developer.apple.com/library/mac/documentation/userexperience/conceptual/applehiguidelines/KeyboardShortcuts/KeyboardShortcuts.html
https://developer.apple.com/library/mac/documentation/userexperience/conceptual/applehiguidelines/Menus/Menus.html