I don't think this is a recent regression. This could also observed in M62.
It seems to be dependent on user's profile, but still unsure.

No repro on:
- Chromebook Pixel 2 / 62.0.3202.97 / 9901.77.0 / Google_Samus.6300.174.0 / regular user
- Chromebook Pixel 2 / 62.0.3202.97 / 9901.77.0 / Google_Samus.6300.174.0 / Guest Mode
- Chromebook Plus / 64.0.3281.0 / 10104.0.0 / Google_Kevin.8785.220.0 / regular user
- Chromebook Pixel / 62.0.3182.0 / 9692.0 / Google_Link.2695.1.169 / regular useruser
- Acer Chromebook R11 / 62.0.3202.97 / 9901.77.0 / Google_Cyan.7287.57.125 / regular user
- Acer Chromebook R11 / 62.0.3202.97 / 9901.77.0 / Google_Cyan.7287.57.125 / Guest Mode
Repro on:
- Chromebook Pixel / 62.0.3182.0 / 9692.0 / Google_Link.2695.1.169 / Guest Mode
- Chromebook Plus / 64.0.3281.0 / 10104.0.0 / Google_Kevin.8785.220.0 / Guest Mode
- chrome on ChromeOS (my Linux workstation) / 64.0.3280.0 / with login manager enabled

+oka@
Were there any related changes to QuickView that might affect this?

Routing to PDF component as the buttons are also hosted in the webview.

My list in #2 seems all red herring. I will take a further look.

This happens after hovering mouse pointer above the padding around the webview area of the quick view.
When running on Linux, it looks like the webview appears slowly so that it's hovering on the blank part of the quick view once, thus it sometimes triggers without that movement.
By this reason I think this is likely to be related to Quick View in the Files app.

Deleted: Screencast 2017-11-30 17:49:04.mp4 222 KB

	Screencast 2017-11-30 17:49:04.mp4 222 KB View Download

I found this happens when cross-process-guests flag is enabled.
I confirmed this stops to happen when I disable cross-process-guests at about://flags.

It is happening since this commit point, which seems to have enabled the flag.
https://chromium.googlesource.com/chromium/src/+/a3fd83c8e17555d176368f36a7108ed3c2365691
Reviewed-on: https://chromium-review.googlesource.com/591628

To lfg@: Will you fill in the right Components value and take a look?

+Platform>Apps>FileManager to Components because this is causing regression of the app.
+Internals>Plugins>PDF as a tentative Components, because it sounded relevant to the issue.

Just a note - I don't think this qualifies as a release blocker?

Comment 2 says this is already happening in Chrome 62. I don't think this is RBS. The difference is fairly subtle so now many people will even notice.

I won't have enough time to investigate this in the short term, assigning to James for triage.

I noticed that the files app loads the PDF into the WebView via a blob:chrome-extension://... url, ... is this due to the fact that WebView doesn't allow loading file:// URLs directly?

It seems none of the mouseover cursor transitions work when the PDF is loaded this way, but they do work when the PDF is loaded (via a web URL) into a WebView (e.g. BrowserSample app).

+creis@ since the file:// issue seems relevant here too.

fukino@ - Re c#15 ... would an appview tag make more sense for this use case?

https://developer.chrome.com/apps/tags/appview

Confirmed that disabling cross-process-frames-for-guests stops the regression from occurring.

 Issue 802197  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9fc8f44f974c5103ef70c670e2b5e6d1a4160efc

commit 9fc8f44f974c5103ef70c670e2b5e6d1a4160efc
Author: W. James MacLean <wjmaclean@chromium.org>
Date: Tue Jan 23 21:23:40 2018

RenderWidgetHostViewGuest was never wired to CursorManager.

The CL updates RWHVG to use CursorManager when updating its
cursor.

Bug:  775488 
Change-Id: I5534aa04c78d27db2d22ccc4134c66fa1f9ea4af
Reviewed-on: https://chromium-review.googlesource.com/879322
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531351}
[modify] https://crrev.com/9fc8f44f974c5103ef70c670e2b5e6d1a4160efc/content/browser/browser_plugin/browser_plugin_guest.cc
[modify] https://crrev.com/9fc8f44f974c5103ef70c670e2b5e6d1a4160efc/content/browser/frame_host/render_widget_host_view_guest.cc
[modify] https://crrev.com/9fc8f44f974c5103ef70c670e2b5e6d1a4160efc/content/browser/frame_host/render_widget_host_view_guest.h

This should be fixed as of the CL in c#20 ... please reopen if this bug is observed after r531351.

After this bakes a day or two we'll request merge to M65. I suspect this is not critical enough to try and get into M64.

Your change meets the bar and is auto-approved for M65. Please go ahead and merge the CL to branch 3325 manually. Please contact milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), bhthompson@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fe970f88cf8965b42c2601891cb2d7237d3bc122

commit fe970f88cf8965b42c2601891cb2d7237d3bc122
Author: W. James MacLean <wjmaclean@chromium.org>
Date: Fri Jan 26 15:47:47 2018

RenderWidgetHostViewGuest was never wired to CursorManager.

The CL updates RWHVG to use CursorManager when updating its
cursor.

TBR=wjmaclean@chromium.org

(cherry picked from commit 9fc8f44f974c5103ef70c670e2b5e6d1a4160efc)

Bug:  775488 
Change-Id: I5534aa04c78d27db2d22ccc4134c66fa1f9ea4af
Reviewed-on: https://chromium-review.googlesource.com/879322
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#531351}
Reviewed-on: https://chromium-review.googlesource.com/889180
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#115}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
[modify] https://crrev.com/fe970f88cf8965b42c2601891cb2d7237d3bc122/content/browser/browser_plugin/browser_plugin_guest.cc
[modify] https://crrev.com/fe970f88cf8965b42c2601891cb2d7237d3bc122/content/browser/frame_host/render_widget_host_view_guest.cc
[modify] https://crrev.com/fe970f88cf8965b42c2601891cb2d7237d3bc122/content/browser/frame_host/render_widget_host_view_guest.h