byebug: Thread hanging when used with Capybara.

When using pry-byebug to try to debug some feature tests, I’ve run into what I believe is a deadlock. Whenever I try to use Capybara’s finder API my thread hangs indefinitely. I saw this deivid-rodriguez/pry-byebug#69 and tried removing pry-byebug to no avail. I did some digging and it looks to be with byebug itself. As per your request from the referenced thread, I’m creating this new issue in the byebug repo.

System Details

ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14] rbenv 0.4.0 OSX Yosemite 10.10.5

Tested Versions

This issue sprung up somewhat recently (within the last month or so), but I’ve just worked around it. In light of this, I tried running byebug versions 7.0.0 up through the latest on master but the issue remained the same.

Reproduction

I’ve created a sample application with a minimal reproduction: https://github.com/donald-s/byebug_bug.

1. Clone the repo
2. Install Qt 5 (if you don’t already have it) for capybara-webkit
3. Run bundle install
4. Run rspec spec/features/home_spec.rb and watch it pass (ensuring no issues with Capybara)
5. Finally, uncomment byebug , run the test again with the above command, and run page.find('h1') when the debugger opens.

Expected Behavior

Capybara returns an object or raises an error.

Observed Behavior

Thread running byebug hangs indefinitely.

I removed byebug and pry-byebug from the repo I originally discovered this in and things work as expected.