Should `revert-buffer' preserve text-scaling by default?

[Karl Fogel]

Currently, when you do `revert-buffer' it will discard any text scale changes 
done in that buffer (i.e., done via "C-x C-+" / `text-scale-adjust').  

The effect -- unexpected to me at least -- is that when I revert a buffer whose 
text size I have increased, the text suddenly shrinks down again (to whatever 
the default text scale was).

Before I "fix" this, I'd like to know if others consider it a bug or a feature.

The reason the text scale currently gets reverted is that text-scaling happens 
to be implemented via the minor mode `text-scale-mode'.  Since `revert-buffer' 
doesn't preserve modes by default, text-scaling is lost when one reverts.  
There is an optional `preserve-modes' argument to `revert-buffer', but I don't 
think that just making wrapper that sets that argument to non-nil is a good 
solution here, because that would preserve *all* the modes.  From the user's 
point of view, the fact that text-scaling happens to be implemented via a minor 
mode is irrelevant and is probably not something most users are even aware of.  
Text-scaling is more like a magnifying glass (or reducing glass) that one 
places over the text of a particular buffer.  Reverting the buffer to its 
original content is something that happens "under" the glass -- there's no 
reason reverting should change the magnification/reduction level.

We could fix this by making `revert-buffer--default' preserve `text-scale-mode' 
and `text-scale-amount', either unconditionally or conditionally based on a new 
variable `revert-buffer-preserve-text-scale'.  I would be fine with either 
solution; if conditional, I would opt for `revert-buffer-preserve-text-scale' 
being true by default.  (Note that if someone has written a custom 
`revert-buffer-function' to be used instead of `revert-buffer--default', then 
we should let their custom function decide what to do about text-scaling; it 
can choose whether or not to honor a `revert-buffer-preserve-text-scale' 
variable.)

Before I go down this road, I'd like to know if anyone thinks text-scaling is 
just another minor mode and should be bound together with other modes via the 
existing `preserve-modes' flag to `revert-buffer'.  I think the opposite: that 
is, I think text-scaling should be treated specially, for the reasons given 
above.  I just wanted to check here before writing any code.

Best regards,
-Karl