Reduced with m-c:
BuildID=20190225215823
SourceStamp=49b2a4c8be018f92d050512f9646cb3004ec1bec

Hit MOZ_CRASH(assertion failed: child_index.0 > parent_index.0) at gfx/wr/webrender/src/clip_scroll_tree.rs:226

#0 MOZ_Crash(char const*, int, char const*) src/obj-firefox/dist/include/mozilla/Assertions.h:314:3
#1 GeckoCrash src/toolkit/xre/nsAppRunner.cpp:5220:3
#2 gkrust_shared::panic_hook::h28d938b83ed40eee src/toolkit/library/rust/shared/lib.rs:234:8
#3 core::ops::function::Fn::call::h55b6e9fae1032cf0 /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/ops/function.rs:78:4
#4 std::panicking::rust_panic_with_hook::h8cbdfe43764887be /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:495:16
#5 std::panicking::begin_panic::h1430648ed59e7de0 /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:425:4
#6 webrender::clip_scroll_tree::ClipScrollTree::map_rect_to_parent_space::h0331e5be52942347 src/gfx/wr/webrender/src/clip_scroll_tree.rs:226:8
#7 webrender::batch::AlphaBatchBuilder::add_prim_to_batch::ha836bc369727d6d3 (.llvm.4043317697329123591) src/gfx/wr/webrender/src/batch.rs:1490:48
#8 webrender::batch::AlphaBatchBuilder::add_pic_to_batch::h54b3a67e020345ad src/gfx/wr/webrender/src/batch.rs:560:12
#9 webrender::batch::AlphaBatchBuilder::add_prim_to_batch::ha836bc369727d6d3 (.llvm.4043317697329123591) src/gfx/wr/webrender/src/batch.rs:1592
#10 webrender::batch::AlphaBatchBuilder::add_pic_to_batch::h54b3a67e020345ad src/gfx/wr/webrender/src/batch.rs:560:12
#11 webrender::batch::AlphaBatchBuilder::add_prim_to_batch::ha836bc369727d6d3 (.llvm.4043317697329123591) src/gfx/wr/webrender/src/batch.rs:1592
#12 webrender::batch::AlphaBatchBuilder::add_pic_to_batch::h54b3a67e020345ad src/gfx/wr/webrender/src/batch.rs:560:12
#13 webrender::batch::AlphaBatchBuilder::add_prim_to_batch::ha836bc369727d6d3 (.llvm.4043317697329123591) src/gfx/wr/webrender/src/batch.rs:1216
#14 webrender::batch::AlphaBatchBuilder::add_pic_to_batch::h54b3a67e020345ad src/gfx/wr/webrender/src/batch.rs:560:12
#15 webrender::batch::AlphaBatchBuilder::add_prim_to_batch::ha836bc369727d6d3 (.llvm.4043317697329123591) src/gfx/wr/webrender/src/batch.rs:1592
#16 webrender::batch::AlphaBatchBuilder::add_pic_to_batch::h54b3a67e020345ad src/gfx/wr/webrender/src/batch.rs:560:12
#17 _$LT$webrender..tiling..ColorRenderTarget$u20$as$u20$webrender..tiling..RenderTarget$GT$::build::h4539ad528d445034 src/gfx/wr/webrender/src/tiling.rs:434
#18 webrender::tiling::RenderPass::build::hc785189a3cf30727 src/gfx/wr/webrender/src/tiling.rs:976:16
#19 webrender::frame_builder::FrameBuilder::build::hc338d596cd133208 src/gfx/wr/webrender/src/frame_builder.rs:629:16
#20 webrender::render_backend::Document::build_frame::h82b762a1497b9dd7 src/gfx/wr/webrender/src/render_backend.rs:516:24
#21 webrender::render_backend::RenderBackend::update_document::hd15e22410d421961 src/gfx/wr/webrender/src/render_backend.rs:1376:40
#22 webrender::render_backend::RenderBackend::prepare_transaction::h5e07428e482587f1 src/gfx/wr/webrender/src/render_backend.rs:1252:12
#23 webrender::render_backend::RenderBackend::process_api_msg::h65283a3eeb682c5d src/gfx/wr/webrender/src/render_backend.rs:1180
#24 webrender::render_backend::RenderBackend::run::h2f2194afc07aa209 src/gfx/wr/webrender/src/render_backend.rs:937:20
#25 webrender::renderer::Renderer::new::_$u7b$$u7b$closure$u7d$$u7d$::hea880930eb5474ef src/gfx/wr/webrender/src/renderer.rs:1961:12
#26 std::sys_common::backtrace::__rust_begin_short_backtrace::h184a46d409a52843 /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/sys_common/backtrace.rs:136
#27 std::thread::Builder::spawn_unchecked::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::h778e94b6e78d14bd /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/thread/mod.rs:477:16
#28 _$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$$LP$$RP$$GT$$GT$::call_once::h951bb1bb22870fc7 /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panic.rs:319
#29 std::panicking::try::do_call::h51d5d901663c07c0 (.llvm.5437706153719687354) /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:310
#30 __rust_maybe_catch_panic /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libpanic_abort/lib.rs:39:4

https://crash-stats.mozilla.org/report/index/de938a63-61c4-488c-bbf4-b1a170190226#tab-details

Cant repro the crash anymore.
Can you still reproduce?

(In reply to Mayank Bansal from comment #2)

Can you still reproduce?

I am also unable to reproduce this with the attached test case.

The fuzzers have not seen this issue since mid March.