Generation of PDF hanging when the window won't close

[jyurek]

I am trying to generate a PDF from a page in my app via ferrum (by way of ferrum_pdf). When running my script with FERRUM_DEBUG=1 set, I get expected data right up until this happens:

[...]

▶ 5.411350000000311 {"method":"Page.printToPDF","params":{"landscape":false,"paperWidth":8.5,"paperHeight":11,"scale":1.0,"encoding":"binary","transferMode":"ReturnAsStream"},"id":17,"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.433713000000353 {"method":"CSS.mediaQueryResultChanged","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.433773000000656 {"method":"CSS.mediaQueryResultChanged","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.433875000000626 {"method":"CSS.fontsUpdated","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.4339230000005045 {"method":"DOM.scrollableFlagUpdated","params":{"nodeId":2,"isScrollable":true},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.433964000000742 {"method":"CSS.mediaQueryResultChanged","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.4340000000001965 {"method":"CSS.fontsUpdated","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.434056000000055 {"method":"CSS.mediaQueryResultChanged","params":{},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.4340799999999945 {"method":"DOM.scrollableFlagUpdated","params":{"nodeId":2,"isScrollable":true},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.483807000000525 {"id":17,"result":{"data":"","stream":"1"},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}


▶ 5.483986000000186 {"method":"IO.read","params":{"handle":"1","size":131072},"id":18,"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.486088999999993 {"id":18,"result":{"base64Encoded":true,"data":"[...VALID PDF DATA...]","eof":false},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}


▶ 5.486253000000033 {"method":"IO.read","params":{"handle":"1","size":131072},"id":19,"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.491773000000649 {"id":19,"result":{"base64Encoded":true,"data":"","eof":true},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}


▶ 5.4919040000004316 {"method":"Target.closeTarget","params":{"targetId":"B680452C22BC38666756304B94FAE641"},"id":20,"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.493644000000131 {"id":20,"result":{"success":true},"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8"}
    ◀ 5.493747999999869 {"method":"Target.targetInfoChanged","params":{"targetInfo":{"targetId":"B680452C22BC38666756304B94FAE641","type":"page","title":"Shepherd","url":"about:blank","attached":false,"canAccessOpener":false,"browserContextId":"32E0DF7CE917C6F46D2FF8ABB7771CD0"}}}
    ◀ 5.494045000000369 {"method":"Target.detachedFromTarget","params":{"sessionId":"93FF304962AB15CD3FF17CEF057BB1B8","targetId":"B680452C22BC38666756304B94FAE641"}}
    ◀ 5.494136000000253 {"method":"Target.targetDestroyed","params":{"targetId":"B680452C22BC38666756304B94FAE641"}}
    ◀ 5.494684000000234 {"method":"Target.targetDestroyed","params":{"targetId":"347BE33CE30CE4A256D23AD6117F7F47"}}
    ◀ 5.496049000000312 {"method":"Target.targetDestroyed","params":{"targetId":"829BA5E45516DA870EF53451E799FB76"}}
F

The F is because I'm running this via RSpec and the test is marked as failed. The error message in the test is reported as unknown error: failed to close window in 20 seconds

Expected outcome:

The window should close and the test should pass (nothing else happens in the test).