Bugs like these are pervasive in languages like Java that give no protection against even the most basic race condition causes. It’s nearly impossible to write reliable concurrent code. Freya only helps if you actually use it to test everything which is not realistic. I am convinced, after my last year long struggle to get a highly concurrent Java (actually Kotlin but Kotlin does not add much to help) module at work, that we should only use languages that provide safe concurrency models, like Erlang/Elixir and Rust, or actor-like like Dart and JavaScript, where concurrency is required.

without reworking of the code all these checks of the executor and queue state and queue manipulations have to be under a mutex, and that is just a few lines.

The light mode is fine, but you're right the dark mode is truly awful, the code blocks are unreadable.

edit: for some reason the author overrode the background color on code blocks via an inline style of

    background-color:#f0f0f0

from

    var(--code-background-color) = #f2f2f2

to make the background nigh imperceptibly darker, but then while the stylesheet properly switches the to #01242e in dark mode the inline override stays and blows it to bit.

Not that it's amazing if you remove the inline stle, on account of operators and method names being styled pretty dark (#666 and #4070a0).