Import selection rectangle leaves trails visible in screenshot

[Rickard Westman]

Under Kubuntu 18.04.2 LTS, the import tool leaves remnants of the selection rectangle in the captured screenshot. Those are also visible in real-time when moving the mouse to adjust the size of the selection rectangle, as if the old-sized rectangle is not always erased before the new-sized rectangle is drawn. There also seems to be some kind of double buffering going on (in the KDE compositor?) so that the display alternates between multiple sets of rectangle remnants. Speculating a bit, perhaps the double buffering is also the root cause for the problem - maybe the old-sized rectangle *is* erased but in the wrong buffer.

The standard screenshot tool in KDE (accessed using the PrtScr key) does not exhibit the problem using its equivalent mode (rectangular selection).

Output from lsb_release -rd:

Description: Ubuntu 18.04.2 LTS
Release: 18.04

Output from apt-cache policy imagemagick-6.q16:

imagemagick-6.q16:
  Installed: 8:6.9.7.4+dfsg-16ubuntu6.7
  Candidate: 8:6.9.7.4+dfsg-16ubuntu6.7
  Version table:
 *** 8:6.9.7.4+dfsg-16ubuntu6.7 500
        500 http://se.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     8:6.9.7.4+dfsg-16ubuntu6 500
        500 http://se.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

What I expected to happen:

Obtaining a clean screenshot of the region corresponding to the selection rectangle (as in attachment "emacs_screenshot_kde.png").

What happened instead:

Obtained a screenshot including remnants from the selection rectangle (see attachment "emacs_screenshot_imagemagick.png").

[Rickard Westman (rwestman)]

This seems to be the same bug, reported by someone else a little more than a year ago:

https://imagemagick.org/discourse-server/viewtopic.php?f=3&t=33972&p=156057&hilit=import#p156057

[Bill Yikes (yik3s)]

I've noticed that this bug manifests whenever a the screen is animated. And sometimes the screen /appears/ ot be still, but perhaps some crappy javascript does something like repeated refreshing to make the still image "animated".

A good reproducer: use a browser other than Tor Browser to access a Cloudflared website like opensecrets.org, and do so over Tor. In this case, Cloudflare will push a CAPTCHA. Using import on that captcha always consistently results in trails of selection rectangles.

When the screen is truly still, the import command works fine, generally. In any case, I concur that this bug certainly exists.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.