As described by the reporter.

STR:

1. go to a bug on bugs.chromium.org (e.g. https://bugs.chromium.org/p/chromium/issues/detail?id=997540&q=&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified)
2. try to select the text of the description with the mouse by slowly dragging down or up

ER:
text gets selected without glitches

AR:
when the cursor is between lines, the selection jumps around wildly.

A Video Of the Bug in Action is here:

https://streamable.com/1zbng

while the bug was reproduced in a VM (for eliminating variables), the same happens on bare metal.

This is happening when the cursor is crossing the in-between area of the two lines.
It's a lot easier to when we draw outlines and magnify the content. See the screenshot.

I could not reproduce this bug on 59, it looks like a regression.

Hi Andrei, could you find someone help to get the regression window? Thanks.

Regression window:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=48a5d87cf9bdb59e39653ab331df04ea2a04267d&tochange=20d536fd0f2a02bd4527044d367cf98bebbb358d

Regressed by: 20d536fd0f2a02bd4527044d367cf98bebbb358d Olli Pettay — Bug 1460069 - enable Shadow DOM in Nightly, r=emilio

(In reply to Edgar Chen [:edgar] from comment #2)

Hi Andrei, could you find someone help to get the regression window? Thanks.

Looks like Alice0075 White already took care of this, thank you! Edgar, if there's something else that QA should help with here, please feel free to needinfo me again.

Minimum test script for reference.

It seems caused by the GetSelectionClosestFrame() call in https://searchfox.org/mozilla-central/rev/efdf9bb55789ea782ae3a431bda6be74a87b041e/layout/generic/nsFrame.cpp#5431 which returns the first <br>'s frame in the shadow dom when the mouse cursor moves out from shadow dom.