User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

1. Make sure the pref gfx.webrender.enable-item-cache is turns on.
2. Open the testcase.html
3. Randomly select text on the page.

Actual results:

The text of the paragraph with Emoji is shaded.

Expected results:

There should be no shaded.

Sorry for my bad English.

2020-05-24T16:11:14.242000: INFO : application_version: 76.0a1
2020-05-24T16:11:14.242000: INFO : platform_buildid: 20200319084853
2020-05-24T16:11:14.242000: INFO : platform_changeset: 8f4b47079a44eeea87caa560b3b072148551aa3c

Regression window:

2020-05-24T16:11:14.242000: INFO : platform_repository: https://hg.mozilla.org/integration/autoland
2020-05-24T16:11:14.242000: INFO : platform_version: 76.0a1
2020-05-24T16:11:33.717000: INFO : Narrowed integration regression window from [2d55f2c0, 97879e1e] (3 builds) to [8f4b4707, 97879e1e] (2 builds) (~1 steps left)
2020-05-24T16:11:33.726000: DEBUG : Starting merge handling...
2020-05-24T16:11:33.726000: DEBUG : Using url: https://hg.mozilla.org/integration/autoland/json-pushes?changeset=97879e1e1dcb4c0577bb4ed4b7f803a9c236fc22&full=1
2020-05-24T16:11:35.898000: DEBUG : Found commit message:
Bug 1616412 - Enable WebRender display item caching r=kvark

Differential Revision: https://phabricator.services.mozilla.com/D63269

2020-05-24T16:11:35.898000: DEBUG : Did not find a branch, checking all integration branches
2020-05-24T16:11:35.898000: INFO : The bisection is done.
2020-05-24T16:11:35.914000: INFO : Stopped

Thanks for the report!

KDE, X11, Debian Testing: If I select bottom-left "🤣the" by double click, this line's black text changes into the grey of lines without emoji.
Disabling gfx.webrender.enable-item-cache fixes the bug.

bug 1558926 introduced gfx.webrender.enable-item-cache on 2020-01-27: unaffected

mozregression --good 2020-01-28 --bad 2020-03-28 --pref gfx.webrender.all:true gfx.webrender.enable-item-cache:true security.sandbox.content.level:0 -a https://bugzilla.mozilla.org/attachment.cgi?id=9151321

12:49.44 INFO: Last good revision: a1a54431e6d61a5c12a69c3eb799ef59b088455e
12:49.44 INFO: First bad revision: cac023f94cda80ae5f328d1ef2fdb2194b82cb1e
12:49.44 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=a1a54431e6d61a5c12a69c3eb799ef59b088455e&tochange=cac023f94cda80ae5f328d1ef2fdb2194b82cb1e

cac023f94cda80ae5f328d1ef2fdb2194b82cb1e Miko Mynttinen — Bug 1614655 - Part 3: Retain WR display items for nsDisplayBorder, nsDisplayText, nsDisplayButtonBorder, and nsDisplayButtonForeground r=jrmuizel
aaed31bbe8f4e0a8892e632633c27033549eaf33 Miko Mynttinen — Bug 1614655 - Part 2: Allow 1:n mapping of Gecko - WR display items r=jrmuizel
0e791acac783e4ef9728fd2feded2d405010a76d Miko Mynttinen — Bug 1614655 - Part 1: Remove item_key from CommonItemProperties r=jrmuizel

I ran mozregression on Windows 10, and the bug reproduces all the way up to 2016, when the emojis were enabled (bug 1286011). I filed bug 1640635 for this.

If |nsDisplayItem::CreateWebRenderCommands()| returns false, the items that have been pushed to |DisplayListBuilder::pending_chunk| need to be cleared, because the item will be pushed as an image instead. This happens, for example, when nsDisplayText encounters unsupported features.

https://hg.mozilla.org/mozilla-central/rev/d9f5353bcddc