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If you place ExplSyntax on/off inside tikzset, code (that is expected to work) within ExplSyntax would generate errors. But if you bring ExplSyntax outside of tikzset (enclosing the tikzset), it makes tikzset behave unexpectedly. How can I use ExplSyntax inside tikzset? Or do I need to (disappointingly, yet again) search for work arounds?

Block within ExplSyntax generates an error inside tikzset:

documentclass[varwidth]standalone

usepackageexpl3
usepackagetikz

ExplSyntaxOn
 int_new:NmyCounter
 int_set:NnmyCounter0
ExplSyntaxOff

tikzset
 every node/.style=
 /utils/exec=
 ExplSyntaxOn
 % following line generate an error
 % int_incr:NmyCounter
 ExplSyntaxOff
 
 
 

begindocument

 begintikzpicture
 path nodesome text;
 endtikzpicture

enddocument

Although block within ExplSyntax does not generate an error, it does not increment myCounter, causes labels and positioning commands (who knows what else) within tikzset to generate errors, and prevents a standard counter (myStandardCounter) from being incremented:

documentclass[varwidth]standalone

usepackageexpl3
usepackagetikz

newcountermyStandardCounter

% let's bring ExplSyntax outside of tikzset and try again
ExplSyntaxOn
 int_new:NmyCounter
 int_set:NnmyCounter0
 tikzset
 every node/.style=
 /utils/exec=
 % neither of the counters below are incremented
 int_incr:NmyCounter
 stepcountermyStandardCounter
 
 
 
ExplSyntaxOff

begindocument

 begintikzpicture
 path nodesome text;
 endtikzpicture

 % both counters are at their initial values (zero)

 ExplSyntaxOn
 expl3~counter:~int_use:NmyCounter
 ExplSyntaxOff

 standard counter: thevaluemyStandardCounter

enddocument

First, catcode changes (ExplSyntaxOn/makeatletter, ExplSyntaxOff/makeatother, and the like) will not work inside macro arguments. TeX turns a input character into a so-called token when it first sees that character. When you do:

tikzset
 
 /utils/exec=
 
 ExplSyntaxOn
 int_incr:N myCounter
 ExplSyntaxOff
 
 

The tikzset macro grabs everything between and as argument and, at this point, TeX “sees” those characters, which are turned into tokens using the current catcode regime (with ExplSyntaxOff), so int_incr:N myCounter is tokenized as int _ i n c r : N myCounter, and this tokenization is “frozen”, which means you can't change it.

After that the /utils/exec key does its thing and ExplSyntaxOn is executed, but at this point it's too late, so it won't work.

Second, using your last code with ExplSyntaxOn outside. Remember that in the expl3 syntax spaces are ignored, so the every node/.style key is actually everynode/.style. And in TikZ spaces do matter. You need every~node/.style.

And last, you execute the node code inside a tikzpicture environment, and this creates a bunch of grouping levels, so any local assignment is lost when the endtikzpicture is reached. You need to use int_gincr:N to make it work (LaTeX's stepcounter uses global assignments).

Output:

Working code:

documentclass[varwidth]standalone

usepackageexpl3
usepackagetikz

newcountermyStandardCounter

% let's bring ExplSyntax outside of tikzset and try again
ExplSyntaxOn
 int_new:N myCounter
 int_gset:Nn myCounter 0
 tikzset
 every~node/.style=
 /utils/exec=
 % neither of the counters below are incremented
 int_gincr:N myCounter
 stepcountermyStandardCounter
 
 
 
ExplSyntaxOff

begindocument

 begintikzpicture
 path nodesome text;
 endtikzpicture

 % both counters are at their initial values (zero)

 ExplSyntaxOn
 expl3~counter:~int_use:NmyCounter
 ExplSyntaxOff

 standard counter: thevaluemyStandardCounter

enddocument