https://github.com/streamlit/streamlit/issues/1440
It's over a month old now, with multiple people reporting the same bug and iterating on the problem with no apparent narrowing towards a reason for why this happens, let alone a solution.
As one of our engineers mentions in the linked issue, this bug is hard to trigger (as is the nature of many bugs), and she is working on a solution[1]. This issue has also been reported here[2], with the solution of adding time.sleep(1) inside the loop usually resolving that problem.
[1] https://github.com/streamlit/streamlit/pull/1494 [2] https://discuss.streamlit.io/t/displaying-images-from-opencv...
I know this is a complicated problem domain (because it's the web, Mathematica did this well with complex graphs in the mid-90s), but as it stands now my project has an input on which the graphic depends and a default value; the image breaks before the input control is touched. And it's not a slider, it's a dropdown that makes the app query a db first... I mean, I think I can do this in plain Ajax if I get my story right about sending images. But it's a hobby project meant to explore mathematical ideas...
If you can't do sliders reliably, they should be moved to a beta branch. Streamlit overpromises and underdelivers. But it's a great project, I don't want to be too negative about it.
As I mentioned, there is a pull request already created for this issue; our head of engineering/founder says its being held up by a lack of tests before merging. This problem isn't a slider issue, but a race condition in our session state manager.
Regardless, one month is a pretty trivial amount of time as far as open-source projects go, as well as in the history of Streamlit (project became public Oct 2019). Like all projects, we try and address things in the order of severity, and as far as this issue goes, it's a pretty isolated one, not an indication that the library is unreliable or somehow reflects that our engineers can't design at an early-90s level of engineering.