Testcase found while fuzzing mozilla-central rev 73f050da7d20 (built with --enable-debug).

Assertion failure: childReflowStatus.IsComplete() (We gave flex item unconstrained available block-size, so it should be complete), at /builds/worker/checkouts/gecko/layout/generic/nsFlexContainerFrame.cpp:1924

    #0 0x7f359eec1727 in nsFlexContainerFrame::MeasureAscentAndBSizeForFlexItem(nsFlexContainerFrame::FlexItem&, mozilla::ReflowInput&) /builds/worker/checkouts/gecko/layout/generic/nsFlexContainerFrame.cpp:1922:3
    #1 0x7f359eec80c0 in nsFlexContainerFrame::SizeItemInCrossAxis(mozilla::ReflowInput&, nsFlexContainerFrame::FlexItem&) /builds/worker/checkouts/gecko/layout/generic/nsFlexContainerFrame.cpp:4305:7
    #2 0x7f359eeca2f2 in nsFlexContainerFrame::DoFlexLayout(mozilla::ReflowInput const&, int&, int&, int&, nsTArray<nsFlexContainerFrame::FlexLine>&, nsTArray<nsFlexContainerFrame::StrutInfo>&, nsTArray<nsIFrame*>&, nsFlexContainerFrame::FlexboxAxisTracker const&, int, int, int, bool, ComputedFlexContainerInfo*) /builds/worker/checkouts/gecko/layout/generic/nsFlexContainerFrame.cpp:5008:9
    #3 0x7f359eec89b1 in nsFlexContainerFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) /builds/worker/checkouts/gecko/layout/generic/nsFlexContainerFrame.cpp:4430:5
    #4 0x7f359eeaf540 in nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, mozilla::WritingMode const&, mozilla::LogicalPoint const&, nsSize const&, nsIFrame::ReflowChildFlags, nsReflowStatus&, nsOverflowContinuationTracker*) /builds/worker/checkouts/gecko/layout/generic/nsContainerFrame.cpp:1082:14
    #5 0x7f359ee9e283 in nsCanvasFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) /builds/worker/checkouts/gecko/layout/generic/nsCanvasFrame.cpp:789:7
    #6 0x7f359eeaf540 in nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, mozilla::WritingMode const&, mozilla::LogicalPoint const&, nsSize const&, nsIFrame::ReflowChildFlags, nsReflowStatus&, nsOverflowContinuationTracker*) /builds/worker/checkouts/gecko/layout/generic/nsContainerFrame.cpp:1082:14
    #7 0x7f359eee8be6 in nsHTMLScrollFrame::ReflowScrolledFrame(mozilla::ScrollReflowInput*, bool, bool, mozilla::ReflowOutput*) /builds/worker/checkouts/gecko/layout/generic/nsGfxScrollFrame.cpp:758:3
    #8 0x7f359eee9679 in nsHTMLScrollFrame::ReflowContents(mozilla::ScrollReflowInput*, mozilla::ReflowOutput const&) /builds/worker/checkouts/gecko/layout/generic/nsGfxScrollFrame.cpp:882:3
    #9 0x7f359eeed677 in nsHTMLScrollFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) /builds/worker/checkouts/gecko/layout/generic/nsGfxScrollFrame.cpp:1280:3
    #10 0x7f359eeaf998 in nsContainerFrame::ReflowChild(nsIFrame*, nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, int, int, nsIFrame::ReflowChildFlags, nsReflowStatus&, nsOverflowContinuationTracker*) /builds/worker/checkouts/gecko/layout/generic/nsContainerFrame.cpp:1122:14
    #11 0x7f359ee70a5b in mozilla::ViewportFrame::Reflow(nsPresContext*, mozilla::ReflowOutput&, mozilla::ReflowInput const&, nsReflowStatus&) /builds/worker/checkouts/gecko/layout/generic/ViewportFrame.cpp:337:7
    #12 0x7f359ed794b6 in mozilla::PresShell::DoReflow(nsIFrame*, bool, mozilla::OverflowChangedTracker*) /builds/worker/checkouts/gecko/layout/base/PresShell.cpp:9668:11
    #13 0x7f359ed82bae in mozilla::PresShell::ProcessReflowCommands(bool) /builds/worker/checkouts/gecko/layout/base/PresShell.cpp:9841:24
    #14 0x7f359ed82174 in mozilla::PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush) /builds/worker/checkouts/gecko/layout/base/PresShell.cpp:4247:11
    #15 0x7f359edee9b1 in FlushPendingNotifications /builds/worker/workspace/obj-build/dist/include/mozilla/PresShell.h:1403:5
    #16 0x7f359edee9b1 in nsDocumentViewer::LoadComplete(nsresult) /builds/worker/checkouts/gecko/layout/base/nsDocumentViewer.cpp:959:16
    #17 0x7f359fda88b1 in nsDocShell::EndPageLoad(nsIWebProgress*, nsIChannel*, nsresult) /builds/worker/checkouts/gecko/docshell/base/nsDocShell.cpp:6407:20
    #18 0x7f359fda829a in nsDocShell::OnStateChange(nsIWebProgress*, nsIRequest*, unsigned int, nsresult) /builds/worker/checkouts/gecko/docshell/base/nsDocShell.cpp:5760:7
    #19 0x7f359fda918f in non-virtual thunk to nsDocShell::OnStateChange(nsIWebProgress*, nsIRequest*, unsigned int, nsresult) /builds/worker/checkouts/gecko/docshell/base/nsDocShell.cpp
    #20 0x7f359b6dc18c in nsDocLoader::DoFireOnStateChange(nsIWebProgress*, nsIRequest*, int&, nsresult) /builds/worker/checkouts/gecko/uriloader/base/nsDocLoader.cpp:1348:3
    #21 0x7f359b6db73a in nsDocLoader::doStopDocumentLoad(nsIRequest*, nsresult) /builds/worker/checkouts/gecko/uriloader/base/nsDocLoader.cpp:954:14
    #22 0x7f359b6d9acc in nsDocLoader::DocLoaderIsEmpty(bool, mozilla::Maybe<nsresult> const&) /builds/worker/checkouts/gecko/uriloader/base/nsDocLoader.cpp:757:9
    #23 0x7f359b6dab7d in nsDocLoader::OnStopRequest(nsIRequest*, nsresult) /builds/worker/checkouts/gecko/uriloader/base/nsDocLoader.cpp:640:5
    #24 0x7f359b6db35c in non-virtual thunk to nsDocLoader::OnStopRequest(nsIRequest*, nsresult) /builds/worker/checkouts/gecko/uriloader/base/nsDocLoader.cpp
    #25 0x7f359a6524b6 in mozilla::net::nsLoadGroup::NotifyRemovalObservers(nsIRequest*, nsresult) /builds/worker/checkouts/gecko/netwerk/base/nsLoadGroup.cpp:616:22
    #26 0x7f359a6539c3 in mozilla::net::nsLoadGroup::RemoveRequest(nsIRequest*, nsISupports*, nsresult) /builds/worker/checkouts/gecko/netwerk/base/nsLoadGroup.cpp:523:10
    #27 0x7f359c09ea41 in mozilla::dom::Document::DoUnblockOnload() /builds/worker/checkouts/gecko/dom/base/Document.cpp:11076:18
    #28 0x7f359c07df60 in mozilla::dom::Document::UnblockOnload(bool) /builds/worker/checkouts/gecko/dom/base/Document.cpp:11006:9
    #29 0x7f359c08e35d in mozilla::dom::Document::DispatchContentLoadedEvents() /builds/worker/checkouts/gecko/dom/base/Document.cpp:7569:3
    #30 0x7f359c0ff046 in applyImpl<mozilla::dom::Document, void (mozilla::dom::Document::*)()> /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:1148:12
    #31 0x7f359c0ff046 in apply<mozilla::dom::Document, void (mozilla::dom::Document::*)()> /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:1154:12
    #32 0x7f359c0ff046 in mozilla::detail::RunnableMethodImpl<mozilla::dom::Document*, void (mozilla::dom::Document::*)(), true, (mozilla::RunnableKind)0>::Run() /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:1201:13
    #33 0x7f359a4af522 in mozilla::SchedulerGroup::Runnable::Run() /builds/worker/checkouts/gecko/xpcom/threads/SchedulerGroup.cpp:146:20
    #34 0x7f359a4b555f in mozilla::RunnableTask::Run() /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:452:16
    #35 0x7f359a4b3baa in mozilla::TaskController::DoExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&) /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:732:26
    #36 0x7f359a4b2c54 in mozilla::TaskController::ExecuteNextTaskOnlyMainThreadInternal(mozilla::detail::BaseAutoLock<mozilla::Mutex&> const&) /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:591:15
    #37 0x7f359a4b2e07 in mozilla::TaskController::ProcessPendingMTTask(bool) /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:375:36
    #38 0x7f359a4b8e46 in operator() /builds/worker/checkouts/gecko/xpcom/threads/TaskController.cpp:122:37
    #39 0x7f359a4b8e46 in mozilla::detail::RunnableFunction<mozilla::TaskController::InitializeInternal()::$_3>::Run() /builds/worker/workspace/obj-build/dist/include/nsThreadUtils.h:534:5
    #40 0x7f359a4ca3f5 in nsThread::ProcessNextEvent(bool, bool*) /builds/worker/checkouts/gecko/xpcom/threads/nsThread.cpp:1200:14
    #41 0x7f359a4d04aa in NS_ProcessNextEvent(nsIThread*, bool) /builds/worker/checkouts/gecko/xpcom/threads/nsThreadUtils.cpp:548:10
    #42 0x7f359add2606 in mozilla::ipc::MessagePump::Run(base::MessagePump::Delegate*) /builds/worker/checkouts/gecko/ipc/glue/MessagePump.cpp:87:21
    #43 0x7f359ad3eb23 in MessageLoop::RunInternal() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:334:10
    #44 0x7f359ad3ea3d in RunHandler /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:327:3
    #45 0x7f359ad3ea3d in MessageLoop::Run() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:309:3
    #46 0x7f359eaa6ac8 in nsBaseAppShell::Run() /builds/worker/checkouts/gecko/widget/nsBaseAppShell.cpp:137:27
    #47 0x7f35a02a1063 in XRE_RunAppShell() /builds/worker/checkouts/gecko/toolkit/xre/nsEmbedFunctions.cpp:902:20
    #48 0x7f359add34e9 in mozilla::ipc::MessagePumpForChildProcess::Run(base::MessagePump::Delegate*) /builds/worker/checkouts/gecko/ipc/glue/MessagePump.cpp:237:9
    #49 0x7f359ad3eb23 in MessageLoop::RunInternal() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:334:10
    #50 0x7f359ad3ea3d in RunHandler /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:327:3
    #51 0x7f359ad3ea3d in MessageLoop::Run() /builds/worker/checkouts/gecko/ipc/chromium/src/base/message_loop.cc:309:3
    #52 0x7f35a02a0c48 in XRE_InitChildProcess(int, char**, XREChildData const*) /builds/worker/checkouts/gecko/toolkit/xre/nsEmbedFunctions.cpp:733:34
    #53 0x559ff0e57d87 in content_process_main /builds/worker/checkouts/gecko/browser/app/../../ipc/contentproc/plugin-container.cpp:57:28
    #54 0x559ff0e57d87 in main /builds/worker/checkouts/gecko/browser/app/nsBrowserApp.cpp:305:18
    #55 0x7f35afa190b2 in __libc_start_main /build/glibc-ZN95T4/glibc-2.31/csu/../csu/libc-start.c:308:16

Bugmon Analysis:
Verified bug as reproducible on mozilla-central 20201211093444-91a89ecd1d3a.
The bug appears to have been introduced in the following build range:

Start: ffa15679931a65e6975408b75828778b1519179d (20200416192951)
End: eb275e9ef6d491a2913ac4d2d5bb0ac63d76f577 (20200416193256)
Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ffa15679931a65e6975408b75828778b1519179d&tochange=eb275e9ef6d491a2913ac4d2d5bb0ac63d76f577

I see what the problem is here...

The existing code set mayNeedNextInFlow to true because
the effectiveContentBSize was so large it was greater
than NS_UNCONSTRAINEDSIZE. We shouldn't even do this
comparison at all if the reflow is unconstrained.

There's a r+ patch which didn't land and no activity in this bug for 2 weeks.
:mats, could you have a look please?
For more information, please visit auto_nag documentation.

https://hg.mozilla.org/mozilla-central/rev/0eb470b7f12e

Bugmon Analysis:
Verified bug as fixed on rev mozilla-central 20210116214312-6083cdb61f42.
Removing bugmon keyword as no further action possible.
Please review the bug and re-add the keyword for further analysis.

:mats, since this bug contains a bisection range, could you fill (if possible) the regressed_by field?
For more information, please visit auto_nag documentation.