User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0

Steps to reproduce:

Just click on the icon of an add-on (in my case it happens with uBlock, uMatrix, https everywhere and video dowloadhelper) to open drop-down-UI. It is reproducable with the add-ons, sometimes it happens with the drop-down-list of the searchfield, too.

Actual results:

Drop-down-UI opens, but in the left top corner a rectangle is displayed transparent ( see attached picture). If hardware acceleration is inactive, everything is fine.

Expected results:

The drop-down-UI should be displayed full.

The Bugbug bot thinks this bug should belong to the 'Core::Graphics' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

It happens also with the drop-down-list of the download-button (blue arrow), but very rarely. I can reproduce it with the "About Mozilla Firefox"-window in about every fourth or fifth try (see screenshot). Then I disabled hardware acceleration and it worked smooth.

Can you please attach your "about:support" information with acceleration enabled?

The system is Kubuntu 20.04 with KDE-Plasma-Version 5.18.5.

As requested, the "about:config" file attached.

Can confirm this for firefox 88.01 opensuse tumbleweed. The content display window remains gray (nothing in it).
Once Hw-acceleration is turned off after starting in --safe-mode, the content is displayed expected (pre- 88.01 update).
Hardware: lenovo t430, 16gb, opensuse tumbleweed 20210512.
lspci output:
00:00.0 Host bridge: Intel Corporation 3rd Gen Core processor DRAM Controller (rev 09)
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)
00:14.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset Family USB xHCI Host Controller (rev 04)
00:16.0 Communication controller: Intel Corporation 7 Series/C216 Chipset Family MEI Controller #1 (rev 04)
00:16.3 Serial controller: Intel Corporation 7 Series/C210 Series Chipset Family KT Controller (rev 04)
00:19.0 Ethernet controller: Intel Corporation 82579LM Gigabit Network Connection (Lewisville) (rev 04)
00:1a.0 USB controller: Intel Corporation 7 Series/C216 Chipset Family USB Enhanced Host Controller #2 (rev 04)
00:1b.0 Audio device: Intel Corporation 7 Series/C216 Chipset Family High Definition Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation 7 Series/C216 Chipset Family PCI Express Root Port 1 (rev c4)
00:1c.1 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset Family PCI Express Root Port 2 (rev c4)
00:1c.2 PCI bridge: Intel Corporation 7 Series/C210 Series Chipset Family PCI Express Root Port 3 (rev c4)
00:1d.0 USB controller: Intel Corporation 7 Series/C216 Chipset Family USB Enhanced Host Controller #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation QM77 Express Chipset LPC Controller (rev 04)
00:1f.2 SATA controller: Intel Corporation 7 Series Chipset Family 6-port SATA Controller [AHCI mode] (rev 04)
00:1f.3 SMBus: Intel Corporation 7 Series/C216 Chipset Family SMBus Controller (rev 04)
02:00.0 System peripheral: Ricoh Co Ltd MMC/SD Host Controller (rev 07)
03:00.0 Network controller: Intel Corporation Centrino Ultimate-N 6300 (rev 3e)

Same issue with 89.0.

Same issue with 90.0.

(In reply to itf2 from comment #8)

Same issue with 90.0.

Can you post your about:support with 90?

(In reply to Lee Salzman [:lsalzman] from comment #9)

(In reply to itf2 from comment #8)

Same issue with 90.0.

Can you post your about:support with 90?

about:support, version 90, raw

The glitch in comment 0 indeed looks familiar, we had similar reports before. Can you try if running FF with MOZ_X11_EGL=1 (or just enabling gfx.x11-egl.force-enabled in about:config) helps?

(In reply to Robert Mader [:rmader] from comment #11)

The glitch in comment 0 indeed looks familiar, we had similar reports before. Can you try if running FF with MOZ_X11_EGL=1 (or just enabling gfx.x11-egl.force-enabled in about:config) helps?

Yes, related to the problem with drop-down-UIs gfx.x11-egl.force-enabled=true fixed the problem (tested with the add-ons given above). It doesn't fix the scattered transparent "About Mozilla Firefox"-window.

Same issue with 91.0.

(In reply to itf2 from comment #12)

It doesn't fix the scattered transparent "About Mozilla Firefox"-window.

That's bug 1502519.

Same issue with 92.0.

(In reply to itf2 from comment #15)

Same issue with 92.0.

Yes, this bug will stay until EGL replaces GLX.
Next step is enabling EGL by default in Nightly: bug 1695933