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sddm flashing issues on nvidia (lubuntu)

Originally filed at https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1895905 Also reported by user at https://discourse.lubuntu.me/t/login-screen-flashes/1634

** background & actual result

Clean install of Lubuntu (a QA-test install) on dell [optiplex] 990 (i7-2600, 16gb, nvidia geforce gt 6600 gt) on first login though, the greeter (sddm) has areas of the screen flashing white/black/white/black.. on an interval like a ":" on a clock between minutes & seconds "12:00"

Photo can be seen at - https://photos.google.com/share/AF1QipPuhRraW1jBt0b0QwwWIw9xQp9q-jlXlAq7Xn1c08wyjkuUlkZ_TgwhG8JJESrwug?key=NUc1NEFNekRIcmFuc3NTR3cxTEF2QWlWSVFXX05B

The (1) session drop down, (2) layout drop down, & (3) user box (picture, my username & password entry area)

In the photograph they all appear as black; they flash WHITE alternatively, and that screen should have no black on it at all.

** expected result

No flashing should occur

** Cards impacted

  • nvidia G72 [GeForce 7300 LE]
  • nvidia geforce 6600gt
  • winfast clone of nvidia 7600gt

** Additional information

This wasn't noticed before Ubuntu 19.10 or the 5.3 kernel (noticed first on i386) The same boxes (6600gt & 7600gt) also have issues with Kubuntu as described with bug report https://bugs.launchpad.net/ubuntu/+source/kubuntu-meta/+bug/1891408, initially filed on 18.04.5 (using the 5.4 HWE kernel/i386) but same issue using 5.8 and hirsute/21.04 alpha/amd64. The issue could be the same, that issue in Kubuntu impacts all of Kubuntu experience, and not just input-areas like sddm. I was asked to file upstream on the sddm bug report, thus this.

For video please see https://photos.google.com/share/AF1QipN0pivvzbpFrQGTWGDHWDp5xix4VI7Nf-vz6o47Xfh65eh9sn8CWoSKVorADrNz3g?key=YTVhbWdRX0xkV2hiUEp4T1RzRHZyWFFSMDNlbzdR reported by user on the Lubuntu discourse site (https://discourse.lubuntu.me/t/login-screen-flashes/1634/6?u=guiverc) Note: that user (7300le) sees characters in the background of the black; I only saw pure black on my two (6600gt & 7600gt)

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