[Bug 1858623] [NEW] VNC outputs garbage in zlib mode

[Artyom]

Public bug reported:

TL;DR: When QEMU is launched with VNC as the output and viewed with a
client that defaults to zlib VNC encoding, the resulting output tends to
accumulate artifacts.

Reproduction:
Launch QEMU (tried with versions 4.2.0 and 4.1.0 on Linux  64bit) with -vnc 
0.0.0.0:0
Connect to it with a VNC client that allows you to select encoding, i.e. 
UltraVNC.
Set encoding to zlib (type 6), 32bit color.
As screen content changes it starts accumulating artifacts. Almost certain to 
appear if you open-close windows over a pattern.
Does not seem to depend on guest used, but easier to reproduce with a GUI.

Looks like this: https://orbides.org/img/vnc.png

It appears to be a deflate glitch of some sort - all of the bad pixels
are generated by length/distance codes. Can't narrow it down any more.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1858623

Title:
  VNC outputs garbage in zlib mode

Status in QEMU:
  New

Bug description:
  TL;DR: When QEMU is launched with VNC as the output and viewed with a
  client that defaults to zlib VNC encoding, the resulting output tends
  to accumulate artifacts.

  Reproduction:
  Launch QEMU (tried with versions 4.2.0 and 4.1.0 on Linux  64bit) with -vnc 
0.0.0.0:0
  Connect to it with a VNC client that allows you to select encoding, i.e. 
UltraVNC.
  Set encoding to zlib (type 6), 32bit color.
  As screen content changes it starts accumulating artifacts. Almost certain to 
appear if you open-close windows over a pattern.
  Does not seem to depend on guest used, but easier to reproduce with a GUI.

  Looks like this: https://orbides.org/img/vnc.png

  It appears to be a deflate glitch of some sort - all of the bad pixels
  are generated by length/distance codes. Can't narrow it down any more.

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