[NTG-context] In LuaMetaTeX, `\read` sometimes doesn't grab the entire balanced text

[witiko@gmail.com]

Dear list,

I noticed that in LuaMetaTeX, `\read` sometimes doesn't grab the entire balanced text.

Consider the following two example files:

#### example.tex

```
\starttext
\newread\example
\openin\example=example.txt
\read\example to\hello
\read\example to\world
\closein\example
\hello\ \world
\stoptext
```

#### example.txt

```
{%
hello}
world
```

Running ConTeXt MkIV with the command `context --luatex example.tex` produces a PDF file with the text "hello world", as expected.

In contrast, running ConTeXt LMTX with the command `context example.tex` produces a PDF file with just the text "world", and a warning that "\end occurred inside a group at level 1". Adding the lines `\show\hello` and `\show\world` before `\stoptext` and rerunning the command `context example.tex` reveals that `\read` only assigned the opening brace `{` rather than the whole balanced text `{hello}` to `\hello`:

```
> \hello=global macro:
{
> \world=global macro:
world
```

Best,
Vit
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