I tried v2 and it works, subject to the following comments.

1. (The biggest issue) For me, the 'obvious' implementation would be a narrow text window using the same font and size as the main window. The canvas introduces the problem of having to calculate the number position instead of letting tk do it for free. On my screen, they are two pixel too low.

The fixed size for numbers can be a problem also. If someone with super eyesight (or a system that displays chars larger than nominal) uses 8 pt type, the bigger numbers are jammed together. If someone with poor eyesight uses a bigger font, the numbers will be too small. (This applies to dialogs too, but that is another issue.)

Breakpoint symbols, for the few people who use them, could be selected from ascii ('|', '>', or '+') or non-ascii symbols. I do not think we need a canvas just to have a graphic for this.

I am also interested in a marginal Text_strip class because I think one might be a possible solution to some of the problems with the Shell.

2. Roger claimed "IDLE does not have a method to broadcast that a font was changed". Roger, as extension writer, had to take Idle as given, whereas we can add a 'font-changed' virtual event.  This seems like a good idea. So would be a 'highlights-changed' event. It appears that editor windows get poked (notified) whenever the preferences Apply or Ok button are hit. Text is blackened and re-colorized even if there is no option change. (Edit a large file like EditorWindow.py to see this.) Adding this event should be a separate issue that this one depends on.

3. To me, the default background for line numbers is way too dark.  Easily changed.

4. There should be a way to toggle line numbers in individual editor windows independently of the default setting for a user. (I agree that the delivered default should be off.)  I would add this to the Format menu.