bug#27103: Should show-trailing-whitespace highlight specified spaces?

[Clément Pit--Claudel]

On 2017-05-28 13:44, Eli Zaretskii wrote:
>> And the contents of the buffer would look like this:
>>
>> → (defun …
>>     …)
>>
>> → (defun …
>>     …)
> 
> But this again asks that the display engine could produce 2 different
> display elements out of the same buffer position, which means it would
> need to scan that buffer position twice.  Right?

Possibly — I don't think I understand this part well.  What happens for 
before-string properties?  Could the same thing happen here?

>> I don't think such a result is currently achievable without overlays (which 
>> are not nearly fast enough for my purposes).
> 
> Are overlays really so slow that you are prepared to jump through such
> hoops?

Definitely :) I need these markers for a literate-programming buffer where 
roughly every other line has a marker.  On a 10k lines file, overlays are 
entirely impractical.
(I guess I could lazily add an remove the overlays, but I'm not too excited 
about reimplementing jit-lock's logic :/)

Clément.

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