Using the per-revision bisect providing the bisect results,
Good build: 55.0.2861.0 (Revision: 418732).
Bad build: 55.0.2862.0 (Revision: 419063).

You are probably looking for a change made after 418939 (known good), but no later than 418940 (first known bad).
CHANGELOG URL:
The script might not always return single CL as suspectas some perf builds might get missing due to failure.
 https://chromium.googlesource.com/chromium/src/+log/5cea1b0cf4328831e583fa66f9fa30c8d075d677..30d21915acf01e3cb5746b1a3cb2e8786c2bd841

From the CL above, assigning the issue to the concern owner --
@kenrb -- Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.

Thank You.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e

commit 7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e
Author: kenrb <kenrb@chromium.org>
Date: Wed Sep 21 21:43:21 2016

Disable direct routing of mouse events for RenderWidgetHostViewGuest

When --isolate-extensions became enabled by default, a consequence is
that it turned on by default direct browser-side routing of mouse events
to webviews, even though RenderWidgetHostViewGuest has not been
modified to support that. This has resulted in multiple input-related
regressions.

This CL disables direct routing to fix those problems. Mouse events
will once again be sent to the embedding renderer process and returned
to the BrowserPluginGuest to be dispatched to the guest.

BUG= 648921 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2354283003
Cr-Commit-Position: refs/heads/master@{#420168}

[modify] https://crrev.com/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e/content/browser/frame_host/render_widget_host_view_guest.cc
[modify] https://crrev.com/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e/content/browser/frame_host/render_widget_host_view_guest.h
[modify] https://crrev.com/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e/content/browser/renderer_host/render_widget_host_input_event_router.cc
[modify] https://crrev.com/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e/content/browser/renderer_host/render_widget_host_view_base.cc
[modify] https://crrev.com/7dfa73aa45e87390c8d20ca31ac69a1cf5a91e0e/content/browser/renderer_host/render_widget_host_view_base.h

Verified this issue on Windows-10 using chrome latest Dev #55.0.2868.0 by following steps mentioned in the original comment. Observed right click context menu is displayed at the right position as expected. Hence adding TE-Verified label.

Deleted: RightClickPdf.mp4 2.7 MB

	RightClickPdf.mp4 2.7 MB View Download

Using a pixel ruler I notice two things:
- The misplaced distance seems to be 50% of the distance of the cursor to the upper left corner (no difference if window is fullscren, maximized or windowed).
- The problem only occurred on my second screen.
Using "Version 53.0.2785.143 Built on Ubuntu , running on Ubuntu 16.04 (64-bit)" fresh install, Asus UX303UB.