#212 Zoom + full-screen xournal = garbage on screen when scrolling

[Alexander Shumakovitch]

OS: Linux, Debian Buster (10.5)
Xournal version: 0.4.8.2016

I use xournal in a full-screen mode as my whiteboard for online lectures. This works great with Blackboard Collaborate, but not so much with Zoom. When scrolling the canvas up, I get extra green lines across the screen. Quick scrolling up-and-down-and-up rusults in even uglier artifacts (see screenshots attached). The only way to get rid of them is to force xournal to redraw the canvas by, say, swithing between workspaces.

The reason for this mess, I believe, is that when Zoom shares a window, it draws a green frame around it (and this frame became thicker with the latest update, grrr). Xournal is apparently unaware of this, and doesn't resize its canvas approprialy. So a few pixels at the bottom end up being filled with garbage.

I would be happy with a quick and dirty workaround, say just artificially reduce the full-screen canvas height by 5 pixels or so, since I need to use xournal almost every day.

Thank you,

 --- Alex.

[Denis Auroux]

Ouch. The bug hasn't affected me, due to sheer luck (see below), but I
had heard a colleague complain about this esp with the Windows version,
and indeed the screenshots are painful. I just replicated your bug and
agree it's nasty.

Xournal has no way of knowing that zoom draws on its window without
following proper protocol for a transparent window -- normally scrolling
xournal should not be affected by a transparent window overlaid by Zoom
over it. I do wonder if there is a rounding bug in the underlying
libraries used by xournal to draw on screen (libgnomecanvas) but since
they're not really maintained anymore, that's not much comfort.

Workaround #1: if Zoom works the same on our systems, it allows you to
share either a window or a whole display. You might think that, if
xournal is fullscreen, then the two should do the same thing. Actually
they don't -- for me, sharing the Xournal window triggers the bug, and
sharing the screen doesn't.

Caveat: sharing the display that contains the fullscreen xournal window,
rather than the xournal window, will also share other transient windows
and notifications that appear on that screen, so this will only work if
you don't need to look at other stuff on the same display while the
screen is been shared. Of note for privacy issues, if you have the zoom
participant list or the chat open on top of the fullscreen xournal, they
will appear in the screenshare, possibly exposing participants' private
chat messages to you to everyone.

Workaround #2: make the xournal window maximized but not fullscreen when
you want to share it. (Meh).

Workaround #3: given the size of your display, you can probably afford
to have the status bar stay at the bottom in fullscreen mode (the think
that shows at the bottom in non-fullscreen mode. (Well, I understand
this takes you farther away from fullscreen, but it's better than green
lines all over the place... and slightly more fullscreen-ish than the
previous workaround) To make this happen, edit ~/.xournal/config in a
text editor (first save your config choices, Options -> Save Preferences
to create this file if needed), and look for the line that should say

interface_fullscreen= main_toolbar pen_toolbar drawarea

Change to
interface_fullscreen= main_toolbar pen_toolbar drawarea statusbar

and restart xournal, now fullscreen mode should have the statusbar at
the bottom; I have checked that on my system this avoids the bug. You
could also try other layouts such as
interface_fullscreen= main_toolbar drawarea pen_toolbar
in case you like that better

Hope this helps. Sorry none of these solutions is perfect...
Denis