Chromium

UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

Example URL:
https://www.bilibili.com/blackboard/bnj2017.html

Steps to reproduce the problem:
1. enter this page and watch video, sometimes happened

What is the expected behavior?
no, even no seek

What went wrong?
web elements not belongs to video rendered in player, Sometimes a part of player, sometimes page header. This element isn't moved to video or copyed in code.

At first time I think it's a special problem in this website.
but when I watching video in Youtube. It happens too and hard to reproduce.
I've tryed the other browsers such as Safari, Firefox, Win Chrome, Edge, but can't reproduce this problem. only in Mac Chrome 56.

Did this work before? Yes Mac Chrome 55 or older never happened(In my using time)

Is it a problem with Flash or HTML5? HTML5

Does this work in other browsers? Yes

Chrome version: 56.0.2924.87  Channel: stable
OS Version: OS X 10.12.1
Flash Version: Shockwave Flash 24.0 r0

Contents of chrome://gpu:
Graphics Feature Status
Canvas: Hardware accelerated
Flash: Hardware accelerated
Flash Stage3D: Hardware accelerated
Flash Stage3D Baseline profile: Hardware accelerated
Compositing: Hardware accelerated
Multiple Raster Threads: Enabled
Native GpuMemoryBuffers: Hardware accelerated
Rasterization: Hardware accelerated
Video Decode: Hardware accelerated
Video Encode: Hardware accelerated
VPx Video Decode: Hardware accelerated
WebGL: Hardware accelerated
WebGL2: Hardware accelerated
Driver Bug Workarounds
add_and_true_to_loop_condition
adjust_src_dst_region_for_blitframebuffer
decode_encode_srgb_for_generatemipmap
disable_framebuffer_cmaa
disable_multimonitor_multisampling
disable_webgl_rgb_multisampling_usage
emulate_abs_int_function
get_frag_data_info_bug
init_two_cube_map_levels_before_copyteximage
msaa_is_slow
pack_parameters_workaround_with_pack_buffer
rebind_transform_feedback_before_resume
regenerate_struct_names
remove_invariant_and_centroid_for_essl3
reset_base_mipmap_level_before_texstorage
rewrite_texelfetchoffset_to_texelfetch
scalarize_vec_and_mat_constructor_args
set_zero_level_before_generating_mipmap
unfold_short_circuit_as_ternary_operation
unpack_alignment_workaround_with_unpack_buffer
unpack_image_height_workaround_with_unpack_buffer
use_intermediary_for_copy_texture_image
use_shadowed_tex_level_params
use_unused_standard_shared_blocks
Problems Detected
Multisampling is buggy on OSX when multiple monitors are connected: 237931
Applied Workarounds: disable_multimonitor_multisampling
Unfold short circuit on Mac OS X: 307751
Applied Workarounds: unfold_short_circuit_as_ternary_operation
Always rewrite vec/mat constructors to be consistent: 398694
Applied Workarounds: scalarize_vec_and_mat_constructor_args
Mac drivers handle struct scopes incorrectly: 403957
Applied Workarounds: regenerate_struct_names
On Intel GPUs MSAA performance is not acceptable for GPU rasterization: 527565
Applied Workarounds: msaa_is_slow
glGenerateMipmap fails if the zero texture level is not set on some Mac drivers: 560499
Applied Workarounds: set_zero_level_before_generating_mipmap
Pack parameters work incorrectly with pack buffer bound: 563714
Applied Workarounds: pack_parameters_workaround_with_pack_buffer
Alignment works incorrectly with unpack buffer bound: 563714
Applied Workarounds: unpack_alignment_workaround_with_unpack_buffer
copyTexImage2D fails when reading from IOSurface on multiple GPU types.: 581777
Applied Workarounds: use_intermediary_for_copy_texture_image
Multisample renderbuffers with format GL_RGB8 have performance issues on Intel GPUs.: 607130
Applied Workarounds: disable_webgl_rgb_multisampling_usage
Mac Drivers store texture level parameters on int16_t that overflow: 610153
Applied Workarounds: use_shadowed_tex_level_params
Limited enabling of Chromium GL_INTEL_framebuffer_CMAA: 535198
Applied Workarounds: disable_framebuffer_cmaa
glGetFragData{Location|Index} works incorrectly on Max: 638340
Applied Workarounds: get_frag_data_info_bug
glResumeTransformFeedback works incorrectly on Intel GPUs: 638514
Applied Workarounds: rebind_transform_feedback_before_resume
glTexStorage* are buggy when base mipmap level is not 0: 640506
Applied Workarounds: reset_base_mipmap_level_before_texstorage
Result of abs(i) where i is an integer in vertex shader is wrong: 642227
Applied Workarounds: emulate_abs_int_function
Rewrite texelFetchOffset to texelFetch for Intel Mac: 642605
Applied Workarounds: rewrite_texelfetchoffset_to_texelfetch
Rewrite condition in for and while loops for Intel Mac: 644669
Applied Workarounds: add_and_true_to_loop_condition
Decode and encode before generateMipmap for srgb format textures on macosx: 634519
Applied Workarounds: decode_encode_srgb_for_generatemipmap
Init first two levels before CopyTexImage2D for cube map texture on Intel Mac 10.12: 648197
Applied Workarounds: init_two_cube_map_levels_before_copyteximage
Insert statements to reference all members in unused std140/shared blocks on Mac: 618464
Applied Workarounds: use_unused_standard_shared_blocks
Tex(Sub)Image3D performs incorrectly when uploading from unpack buffer with GL_UNPACK_IMAGE_HEIGHT greater than zero on Intel Macs: 654258
Applied Workarounds: unpack_image_height_workaround_with_unpack_buffer
adjust src/dst region if blitting pixels outside read framebuffer on Mac: 644740
Applied Workarounds: adjust_src_dst_region_for_blitframebuffer
Mac driver GL 4.1 requires invariant and centroid to match between shaders: 639760, 641129
Applied Workarounds: remove_invariant_and_centroid_for_essl3
Version Information
Data exported   2017/2/15 下午4:28:35
Chrome version  Chrome/56.0.2924.87
Operating system        Mac OS X 10.12.1
Software rendering list version 12.06
Driver bug list version 9.24
ANGLE commit id a4aaa2de57dc
2D graphics backend     Skia/56 bf2d9e02d58ea01f1c239f7e2fc024cba140ccb1
Command Line Args       Chrome.app/Contents/MacOS/Google Chrome --flag-switches-begin --flag-switches-end
Driver Information
Initialization time     47
In-process GPU  false
Sandboxed       true
GPU0    VENDOR = 0x8086, DEVICE= 0x0a2e *ACTIVE*
Optimus false
AMD switchable  false
Driver vendor  
Driver version  10.20.23
Driver date    
Pixel shader version    4.10
Vertex shader version   4.10
Max. MSAA samples       8
Machine model name      Macmini
Machine model version   7.1
GL_VENDOR       Intel Inc.
GL_RENDERER     Intel Iris OpenGL Engine
GL_VERSION      4.1 INTEL-10.20.23
GL_EXTENSIONS   GL_ARB_blend_func_extended GL_ARB_draw_buffers_blend GL_ARB_draw_indirect GL_ARB_ES2_compatibility GL_ARB_explicit_attrib_location GL_ARB_gpu_shader_fp64 GL_ARB_gpu_shader5 GL_ARB_instanced_arrays GL_ARB_internalformat_query GL_ARB_occlusion_query2 GL_ARB_sample_shading GL_ARB_sampler_objects GL_ARB_separate_shader_objects GL_ARB_shader_bit_encoding GL_ARB_shader_subroutine GL_ARB_shading_language_include GL_ARB_tessellation_shader GL_ARB_texture_buffer_object_rgb32 GL_ARB_texture_cube_map_array GL_ARB_texture_gather GL_ARB_texture_query_lod GL_ARB_texture_rgb10_a2ui GL_ARB_texture_storage GL_ARB_texture_swizzle GL_ARB_timer_query GL_ARB_transform_feedback2 GL_ARB_transform_feedback3 GL_ARB_vertex_attrib_64bit GL_ARB_vertex_type_2_10_10_10_rev GL_ARB_viewport_array GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_texture_compression_s3tc GL_EXT_texture_filter_anisotropic GL_EXT_texture_sRGB_decode GL_APPLE_client_storage GL_APPLE_container_object_shareable GL_APPLE_flush_render GL_APPLE_object_purgeable GL_APPLE_rgb_422 GL_APPLE_row_bytes GL_APPLE_texture_range GL_ATI_texture_mirror_once GL_NV_texture_barrier
Disabled Extensions    
Window system binding vendor    
Window system binding version  
Window system binding extensions        
Direct rendering        Yes
Reset notification strategy     0x0000
GPU process crash count 0
Compositor Information
Tile Update Mode        Zero-copy
Partial Raster  Enabled
GpuMemoryBuffers Status
ATC     Software only
ATCIA   Software only
DXT1    Software only
DXT5    Software only
ETC1    Software only
R_8     GPU_READ_CPU_READ_WRITE, GPU_READ_CPU_READ_WRITE_PERSISTENT
RG_88   Software only
BGR_565 Software only
RGBA_4444       Software only
RGBX_8888       Software only
RGBA_8888       GPU_READ, SCANOUT
BGRX_8888       GPU_READ, SCANOUT
BGRA_8888       GPU_READ, SCANOUT, GPU_READ_CPU_READ_WRITE, GPU_READ_CPU_READ_WRITE_PERSISTENT
YVU_420 Software only
YUV_420_BIPLANAR        GPU_READ_CPU_READ_WRITE, GPU_READ_CPU_READ_WRITE_PERSISTENT
UYVY_422        GPU_READ_CPU_READ_WRITE, GPU_READ_CPU_READ_WRITE_PERSISTENT
Log Messages
[33715:775:0215/161723.041160:ERROR:latency_info.cc(163)] : GpuCommandBufferStub::OnAsyncFlush, LatencyInfo vector size 190 is too big.
[33715:3711:0215/162406.823470:ERROR:vt_video_decode_accelerator_mac.cc(634)] : Illegal attempt to decode without IDR. Discarding decode requests until next IDR.

I think it related to MediaSource Extension.
player in this website use MSE
github: https://github.com/Bilibili/flv.js

If you need more video or screenshot, I will try to record youtube's