DevTools test devtools/client/inspector/rules/test/browser_rules_edit-selector_12.js triggers[1] an assertion on Stylo debug builds: Assertion failure: !data->mSheet->IsModified() (should not get marked modified during parsing), at /home/worker/workspace/build/src/layout/style/Loader.cpp:1905 #01: mozilla::css::Loader::SheetComplete [xpcom/ds/nsTArray.h:398] #02: mozilla::css::Loader::ParseSheet [layout/style/Loader.cpp:1811] #03: mozilla::css::SheetLoadData::OnStreamComplete [layout/style/Loader.cpp:1010] #04: nsUnicharStreamLoader::OnStopRequest [netwerk/base/nsUnicharStreamLoader.cpp:99] #05: mozilla::net::HttpChannelChild::DoOnStopRequest [netwerk/protocol/http/HttpChannelChild.cpp:1174] #06: mozilla::net::HttpChannelChild::OnStopRequest [netwerk/protocol/http/HttpChannelChild.cpp:1073] #07: mozilla::net::ChannelEventQueue::FlushQueue [netwerk/ipc/ChannelEventQueue.cpp:65] #08: mozilla::net::ChannelEventQueue::ResumeInternal()::CompleteResumeRunnable::Run() #09: mozilla::SchedulerGroup::Runnable::Run [xpcom/threads/SchedulerGroup.cpp:356] #10: nsThread::ProcessNextEvent [mfbt/ScopeExit.h:111] #11: NS_ProcessNextEvent [xpcom/threads/nsThreadUtils.cpp:480] #12: mozilla::ipc::MessagePump::Run [ipc/glue/MessagePump.cpp:98] #13: MessageLoop::RunInternal [ipc/chromium/src/base/message_loop.cc:327] #14: MessageLoop::Run [ipc/chromium/src/base/message_loop.cc:299] #15: nsBaseAppShell::Run [widget/nsBaseAppShell.cpp:158] #16: XRE_RunAppShell [toolkit/xre/nsEmbedFunctions.cpp:882] #17: mozilla::ipc::MessagePumpForChildProcess::Run [ipc/glue/MessagePump.cpp:270] #18: MessageLoop::RunInternal [ipc/chromium/src/base/message_loop.cc:327] #19: MessageLoop::Run [ipc/chromium/src/base/message_loop.cc:299] #20: XRE_InitChildProcess [toolkit/xre/nsEmbedFunctions.cpp:703] #21: content_process_main [ipc/contentproc/plugin-container.cpp:66] #22: main [browser/app/nsBrowserApp.cpp:288] #23: libc.so.6 + 0x20830 #24: _start [1]: https://treeherder.mozilla.org/logviewer.html#?job_id=120109123&repo=try&lineNumber=14805

This is the same issue as bug 1387933. The problem here is that, when user changes selector via devtools, devtools reparses the stylesheet which includes an @import rule. On Gecko, this reparsing would reuse the existing stylesheet of the @import rule, while on Stylo, we would create a new stylesheet and load it again. And since devtools progressively calls EnsureUniqueInnerOnCSSSheets again, we would mark the new stylesheet dirty before it gets load. Note that I think this is actually an existing issue that if devtools is opened before all stylesheets load, we can hit this assertion in both Gecko and Servo.

Brad, is this still an issue? If so, can you have a look?

I'll confirm it.

It appears this test is currently skipped: http://searchfox.org/mozilla-central/rev/cd82cacec2cf734768827ff85ba2dba90a534c5e/devtools/client/inspector/rules/test/browser.ini#151 If it's working, we should remove the skip annotation.

Comment on attachment 8902441 [details] Bug 1386862: stylo: Turn on the devtools/client/inspector/rules/test/browser_rules_edit-selector_12.js test. https://reviewboard.mozilla.org/r/174032/#review180100

Comment on attachment 8902441 [details] Bug 1386862: stylo: Turn on the devtools/client/inspector/rules/test/browser_rules_edit-selector_12.js test. https://reviewboard.mozilla.org/r/174032/#review180114

Pushed by bwerth@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/4f95bb8143aa stylo: Turn on the devtools/client/inspector/rules/test/browser_rules_edit-selector_12.js test. r=bholley,jryans