Do you know if this used to work with some version of Chrome × Flash?

I remember it should be working fine on two or three versions before the above version I was reporting.

Same issue here, 
MacOS Sierra
MacBook Pro (Retina, Mid 2012) 15.4-inch (2880 x 1800)
2.6 GHz Intel Core i7
8 GB 1600 MHz DDR3
NVIDIA GeForce GT 650M 1024 MB
Intel HD Graphics 4000 1536 MB
NPAPI Plug-in version 23.0.0.162 is installed.
PPAPI Plug-in version 23.0.0.162 is installed.
Chrome Version 54.0.2840.41 beta (64-bit)

Connected my MBP Retina via HDMI to a second/dual monitor (SAMSUNG 1366 x 768)

Using each display as a Separate Space.

Opened up a Flash video stream in a separate Chrome tab on the SAMSUNG dual monitor  and attempted to full screen, but same issue happens (video moves to upper left quadrant, like the canton of a flag, leaving the remainder of the screen white).  I can full screen on my MBP and choose to mirror the display onto the SAMSUNG and it will work just fine, but this defeats the purpose of having dual monitors with separate spaces.  I can also full screen with no problems if the Flash video stream is opened up with Safari.

Recent issue, only within the last month or two, as I was previously able to full screen Flash video just on the dual monitor, in a unique Chrome tab, without any problems.  

Unable to reproduce this scenario, tested in Mac 10.12 OS (Dual monitor) with chrome dev version 54.0.2837.0 and stable version 53.0.2785.143.

Played YouTube, amazon, CNN, BBC  videos and able to see the full screen in 2nd monitor.

Could you please let us know the URL of the webpage if it's related to any particular website? 

Thanks...

I could fullscreen YouTube videos fine (HTML5?), but Flash video streams would not go completely full-screen on my Samsung dual monitor.  As an example, I was attempting to full-screen the live video feed on this website: http://www3.nhk.or.jp/nhkworld/en/live/

Each time I tried to full screen this in Chrome Version 54.0.2840.41 beta/Mac 10.12 OS, the problem would occur.

Able to reproduce this issue on Mac 10.12 on latest chrome Stable version 54.0.2840.59. Issue is broken in M48. Below are the bisect details for the same:

Bisect Info:
===========
Good Build : 48.0.2530.0,  Revision Range (352989)                                           
Bad Build  : 48.0.2531.0 , Revision Range (353185)

Flash bisecting provided only good builds, on manually verifying the CL’s between good and bad build versions:

Change Log: https://chromium.googlesource.com/chromium/src/+log/48.0.2530.0..48.0.2531.0?pretty=fuller&n=10000

Suspecting the below Change Log could be the possible culprit

Change URL: https://chromium.googlesource.com/chromium/src/+/2a09e4bff1e12bd7ea8d572709878f5de7c406f8
Review URL: https://codereview.chromium.org/1387473002

spqchan@- Could you please look into this issue, if it's related to your change?  if not could you please help us to reassign this issue to the right owner.

Unable to reproduce the issue on Windows and Linux (Ubuntu 14.04) OS.

Thanks.!

Continuing to receive this regression error into December; look forward to a fix! 

57.0.2950.4 Dev right now, the issue is still there. 

kkaluri@ 

ccameron@ - any ideas why only 1/4 of the Flash content would be visible?

Usually 1/4 screen issues are because of DPI issues, but that's a wide guess.

It's been over half a year now, any updates?

Checking in on this one again.  I'm still getting the same exact problem fullscreening the USTREAM video feed on this page: http://www.telemolise.com/stream.php

Any update here spqchan?

sdy@ for fullscreen work