Hi fhorschig@! I am not at Google so I don't have access to your internal stats. I dont have a Mac so it is tricky for me to debug this.

From which chromium tag/sha1 did the flakiness start? Do you believe that one of my changes caused it?

Thanks for the quick reaction!
It's hard to say as I assigned the bug to who worked with the test recently and might have some expertise about this area. Your CL https://crrev.com/2817073003 touched it for the first time in tow years, so there is a chance, that you at  know some details or know someone who knows.

The first time it occurred (that I can see) was in revision 538fca1e6cadb17dae1add92c5eda299858ea3f1 but it's likely to have been introduces earlier.

The log diff of the failed test:
--- /b/rr/tmp2sXYfm/w/layout-test-results/plugins/mouse-click-plugin-clears-selection-expected.txt
+++ /b/rr/tmp2sXYfm/w/layout-test-results/plugins/mouse-click-plugin-clears-selection-actual.txt
@@ -1,7 +1,6 @@
 CONSOLE MESSAGE: Blink Test Plugin: initializing
 CONSOLE MESSAGE: Blink Test Plugin: DidChangeFocus(true)
 CONSOLE MESSAGE: Blink Test Plugin: MouseDown at (52,52)
-CONSOLE MESSAGE: Blink Test Plugin: MouseUp at (52,52)
 layer at (0,0) size 800x600
   LayoutView at (0,0) size 800x600
 layer at (0,0) size 800x600

I see! Ok... 

My guess is that the MouseUp event is delayed somewhere. Sometimes it reaches blink in time and sometimes not -> flake. I guess you have to disable it for now...

Let's assign to someone who has more experience of Mac's input handling + mouse events? 
I found https://codereview.chromium.org/2898343002 .
 
shrike@ do you know where this MouseUp could have been lost ?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/765ed10683220896163a799d285ce99567d5bd80

commit 765ed10683220896163a799d285ce99567d5bd80
Author: Friedrich Horschig <fhorschig@chromium.org>
Date: Fri Jul 14 11:35:22 2017

Mark plugins/mouse-click-plugin-clears-selection.html as flaky on Mac10.10

TBR=dcheng@chromium.org
TBR_REASON=Update test expectations
NOTRY=true

Bug: 676229, 742742 
Change-Id: I1aaf376ad020c6a4d8c2aec85714ef627132c7a2
Reviewed-on: https://chromium-review.googlesource.com/571704
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486736}
[modify] https://crrev.com/765ed10683220896163a799d285ce99567d5bd80/third_party/WebKit/LayoutTests/TestExpectations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8187e1c3b6fee2215c75de867a2d90cb8b68484f

commit 8187e1c3b6fee2215c75de867a2d90cb8b68484f
Author: Friedrich Horschig <fhorschig@chromium.org>
Date: Fri Jul 14 13:30:59 2017

Mark plugins/mouse-click-plugin-clears-selection.html as flaky on all Mac platforms

Following Mac10.10, Mac10.11 broke, too.
Mark flaky for all Mac platforms so sheriffs don't have to look for this when it
hits Mac10.9.

TBR=dcheng@chromium.org
TBR_REASON=Update test expectations
NOTRY=true

Bug: 676229, 742742 
Change-Id: Iecda3eb55b891dc35ed47552b43786e80f9f4aa3
Reviewed-on: https://chromium-review.googlesource.com/571788
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486748}
[modify] https://crrev.com/8187e1c3b6fee2215c75de867a2d90cb8b68484f/third_party/WebKit/LayoutTests/TestExpectations

Thanks hugoh!

Taking off sheriff queue since the builder is now longer red due to this

shrike@ feel free to reassign if someone else should look into this test

Moving back to triage.

It seems flakyness comes from dispatching mouse up event.

Could you consider to use chrome.gpuBenchmarking.pointerActionSequence?
see  https://crbug.com/711340  and http://goo.gl/BND75q