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I'm trying to add an external link symbol (from https://tex.stackexchange.com/a/294990/163482) to href but for some reason the spacing after the symbol is wrong when I redefine the href command. What would the right way to do it (and what would be the right amount of space)?

documentclassarticle
usepackagetikz,hyperref
newcommandExternalLink%
 tikz[x=1.2ex, y=1.2ex, baseline=-0.5ex, scale=0.75]% 
 beginscope[x=1ex, y=1ex]
 clip (-0.1,-0.1) 
 --++ (-0, 1.2) 
 --++ (0.6, 0) 
 --++ (0, -0.6) 
 --++ (0.6, 0) 
 --++ (0, -1);
 path[draw=black, line width = 0.5,
 rounded corners=0.5] (0,0) rectangle (1,1);
 endscope
 path[draw=black, line width = 0.5] (0.5, 0.5) -- (1, 1);
 path[draw=black, line width = 0.5] (0.6, 1) -- (1, 1) -- (1, 0.6);
 


letorighrefhref
renewcommandhref[2]orighref#1#2$,$ExternalLink

begindocument
noindent
Some text hrefhttp://google.comlink some more text.\
Some text orighrefhttp://google.comlink$,$ExternalLink some more text.
enddocument

When you write:

Some text orighrefhttp://google.comlink$,$ExternalLink some more text.

the space after ExternalLink is ignored by TeX, because ExternalLink is a control sequence whose name ends with a letter. On the contrary, with:

Some text hrefhttp://google.comlink some more text.

there is a space token after link, and it is not ignored. It contributes an interword space to the horizontal list under construction. That is why you see a larger space in the first case of your example (second case here). In order to get rid of this additional space, you can either write:

Some text hrefhttp://google.comlinksome more text.

or use ignorespaces in your redefinition of href so as to ignore spaces following in the input (ignorespaces expands tokens from the input stream until it finds an unexpandable token that is not a 〈space token〉, according to the TeX grammar):

renewcommand*href[2]orighref#1#2$,$ExternalLinkignorespaces

But actually, you don't want to use any of these suggestions. This is because in your ExternalLink command, there is a most probably unwanted space:

newcommandExternalLink%

...

 path[draw=black, ...] ... ;
 % <------- here, remove the space
}

So, your ExternalLink command unconditionally adds this space after the icon. I believe this is unwanted when the hyperlink is followed by a comma or a period, for instance. Therefore I'd remove this space; then you don't need ignorespaces anymore.

Also note that you don't need to be in math mode in order to use ,; it produces a thinspace when used outside of math mode. So, you can do something like:

renewcommand*href[2]orighref#1#2,ExternalLink

Other suggestion: you may want to include the icon inside the clickable link:

renewcommand*href[2]orighref#1#2,ExternalLink

I used renewcommand* because the arguments of href most probably shouldn't contain par tokens; this makes finding errors easier.

Note: there won't be a line break at the ,, because this macro expands to thinspace in non-math mode, which itself expands to a kern, and there can be a break at a kern only if it's immediately followed by glue—which isn't the case here, as ExternalLink starts with a box: the TikZ picture.

Final remark: your links are going to make TeX work hard on the TikZ picture every time you use this redefined href (i.e., for every hyperlink). Since the icon is always the same, there is a way to save on compilation time: save the picture once inside a box, then use the box whenever you need it. That is very cheap on computing resources. All in all, here is my suggestion (I added a picture with a caption because of our discussion in the comments—the usebox command doesn't need to be protected, as the example shows):

documentclassarticle
usepackagetikz
usepackage[hidelinks]hyperref % 'hidelinks' removes the border around links

newsaveboxExternalLinkIcon

beginlrboxExternalLinkIcon
 begintikzpicture[x=1.2ex, y=1.2ex, baseline=-0.5ex, scale=0.75]
 beginscope[x=1ex, y=1ex]
 clip (-0.1,-0.1)
 --++ (-0, 1.2)
 --++ (0.6, 0)
 --++ (0, -0.6)
 --++ (0.6, 0)
 --++ (0, -1);
 path[draw=black, line width = 0.5,
 rounded corners=0.5] (0,0) rectangle (1,1);
 endscope
 path[draw=black, line width = 0.5] (0.5, 0.5) -- (1, 1);
 path[draw=black, line width = 0.5] (0.6, 1) -- (1, 1) -- (1, 0.6);
 endtikzpicture% <--- important!
endlrbox

letorighrefhref

renewcommand*href[2]%
 orighref#1#2,useboxExternalLinkIcon%


begindocument

noindent
Some text hrefhttp://example.comlink some more text. Spacing is also fine
before a hrefhttp://example.comcomma, as you can see. Periods are fine
too, of course---we don't have to do anything special.

beginfigure[htbp]
 centering
 This is the figure contents.
 captionA caption containing a hrefhttp://example.comhyperlink
endfigure

enddocument