Re-enable JET in the tests

And fix an issue found by the latest version.

Author: JamesWrigley

docs/src/_changelog.md

@@ -9,6 +9,13 @@ Changelog](https://keepachangelog.com).
 
 ## Unreleased
 
+### Changed
+- Replaced JSON.jl with a vendored copy of
+  [JSONX](https://github.com/JuliaIO/JSON.jl/tree/master/vendor) ([#1200]). This
+  lets us remove one more dependency and remove all of the invalidations caused
+  by JSON.jl. Load time is also slightly improved, from ~0.08s to ~0.05s on
+  Julia 1.12.
+
 ## [v1.31.1] - 2025-10-20
 
 ### Added

src/inline.jl

@@ -73,6 +73,10 @@ for mime in ipy_mime
     @eval begin
         function display(d::InlineDisplay, ::MIME{Symbol($mime)}, x)
             kernel = _default_kernel
+            if isnothing(kernel)
+                error("Kernel has not been instantiated, cannot display.")
+            end
+
             flush_all() # so that previous stream output appears in order
             send_ipython(kernel.publish[], kernel,
                          msg_pub(kernel.execute_msg, "display_data",

test/jsonx.jl

@@ -252,7 +252,7 @@ import IJulia: JSONX
 
     @testset "AbstractVector{UInt8} Support" begin
         @test JSONX.parse(Vector{UInt8}("null")) === nothing
-        @test JSONX.parse(Vector{UInt8}("42")) === 42
+        @test JSONX.parse(Vector{UInt8}("42")) === Int64(42)
         @test JSONX.parse(Vector{UInt8}("\"hello\"")) == "hello"
         @test JSONX.parse(Vector{UInt8}("[1,2,3]")) == [1, 2, 3]
         @test JSONX.parse(Vector{UInt8}("{\"a\":1}")) == Dict("a" => 1)

test/runtests.jl

@@ -24,7 +24,6 @@ end
     Aqua.test_all(IJulia; stale_deps=(; ignore=[:Pkg, :Conda]))
 end
 
-# Temporarily disabled because JET doesn't seem to play well with JSON v1
-# @testset "JET.jl" begin
-#     JET.test_package(IJulia; target_defined_modules=true)
-# end
+@testset "JET.jl" begin
+    JET.test_package(IJulia; target_modules=(IJulia,))
+end