I don’t see a link to logs.i3wm.org. Did you follow https://i3wm.org/docs/debugging.html? (In case you actually provided a link to a logfile, please ignore me.)

As mentioned on the mailing list, since we have a bad and a good version, a git bisect would be helpful to figure out which commit actually causes this.

OK, I used git bisect to locate the problem.
Here the output:

29f2510fa9484a5fa235de9a1c5c937292151888 is the first bad commit
commit 29f2510fa9484a5fa235de9a1c5c937292151888
Author: Orestis Floros 
Date:   Mon Oct 1 19:47:33 2018 +0300

    Fix aspect ratio bugs
    
    - ICCCM says: > If a base size is not provided, the minimum size is to
    be used in its place and vice versa.
    i3 didn't obey the "vice versa" part. Min size and base size are both
    saved without replacements in window_update_normal_hints,
    floating_check_size makes the needed replacements if either was not
    provided.
    - Aspect ratio is now saved correctly in manage_window because
    window_update_normal_hints is called.
    - i3 didn't save the aspect ratio if the window conformed the given
    aspect ratio range when handle_normal_hints was called. If the window
    was resized to a size outside of the given bounds, i3 didn't correct it.
    - Aspect ratio now affects only tiling windows, like the rest of the
    normal size hints
    - The aspect ratio calculation is now done without a loop
    
    A real life example of how these changes affect the workflow:
    An mpv window, when playing a video, sets its min == max aspect ratio
    during mapping. i3 ignored these hints. When resized, the window's
    aspect ratio was not preserved. With this commit, resizing floating mpv
    windows will always preserve the aspect ratio.

 include/data.h               |   3 +-
 include/floating.h           |  13 +++--
 include/window.h             |   6 ++
 src/commands.c               |   2 +-
 src/con.c                    |   1 -
 src/floating.c               |  85 ++++++++++++++++++++++------
 src/handlers.c               | 130 ++-----------------------------------------
 src/load_layout.c            |   2 +-
 src/manage.c                 |  34 ++---------
 src/render.c                 |  29 ----------
 src/scratchpad.c             |   2 +-
 src/window.c                 | 121 ++++++++++++++++++++++++++++++++++++++++
 testcases/t/133-size-hints.t | 104 +++++++++++++++++++++++++++-------
 13 files changed, 304 insertions(+), 228 deletions(-)

#3432

Thanks, that's very helpful!

@orestisfl Maybe you can have a look?

Thanks, I can reproduce this specifically with mplayer. Mpv (so you can use it as a workaround) does not have this issue, I'll try to find where the fault lies.

btw both i3 4.16 and awesomewm have this issue with fullscreen mplayer

Yeah, MPlayer is known to be misbehaving, here. I'm fairly sure we discussed this before, and I would guess awesome did as well.

I reverted the behaviour locally, I'll submit a fix soon

There is an UPDATE on the next comment. Please, check it out first.

(I'm keeping the lines below just to have more data for testing)

Hey guys, I'm having the same problem with any program running, but only when the workspace is a new one.

Every time I press $mod+[0-9]:

1 - If the workspace already exists (there's at least one application running there), then the behavior is correct.
2 - If the workspace doesn't exist, the background becomes like a "screenshot" of the previous workspace.

Example:

1 - Running Firefox and Terminator on the workspace 2
2 - Pressed $mod1+7
3 - Pressed $mod1+p (my preference to call dmenu)
4 - Typed "terminal" and hit "Enter"

Here's the log for these operations:

09/05/2020 06:47:50 AM - handlers.c:handle_event:1358 - event type 28, xkb_base 85
09/05/2020 06:47:50 AM - _NET_WM_NAME changed to "i3-dump-log"
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - assignments.c:run_assignments:18 - Checking if any assignments match this window
09/05/2020 06:47:50 AM - x.c:x_push_changes:1164 - -- PUSHING WINDOW STACK --
09/05/2020 06:47:50 AM - x.c:x_push_changes:1237 - PUSHING CHANGES
09/05/2020 06:47:50 AM - con.c:con_border_style_rect:1632 - The border width for con is set to: 5
09/05/2020 06:47:50 AM - con.c:con_border_style_rect:1641 - Effective border width is set to: 5
09/05/2020 06:47:50 AM - con.c:con_border_style_rect:1632 - The border width for con is set to: 5
09/05/2020 06:47:50 AM - con.c:con_border_style_rect:1641 - Effective border width is set to: 5
09/05/2020 06:47:50 AM - x.c:x_push_changes:1330 - ENDING CHANGES
09/05/2020 06:47:50 AM - ipc.c:ipc_send_window_event:1615 - Issue IPC window title event (con = 0x56287ad66f80, window = 0x05e0056e)
09/05/2020 06:47:50 AM - handlers.c:handle_event:1358 - event type 28, xkb_base 85
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - Checking window 0x05e0056e (class Terminator)
09/05/2020 06:47:50 AM - dock status does not match
09/05/2020 06:47:50 AM - assignments.c:run_assignments:18 - Checking if any assignments match this window
09/05/2020 06:47:50 AM - x.c:x_push_changes:1164 - -- PUSHING WINDOW STACK --
09/05/2020 06:47:50 AM - x.c:x_push_changes:1237 - PUSHING CHANGES
09/05/2020 06:47:50 AM - x.c:x_push_changes:1330 - ENDING CHANGES
09/05/2020 06:47:50 AM - handlers.c:handle_event:1358 - event type 28, xkb_base 85
09/05/2020 06:47:50 AM - handlers.c:handle_event:1358 - event type 28, xkb_base 85
09/05/2020 06:47:50 AM - handlers.c:handle_event:1358 - event type 28, xkb_base 85

I'm running i3 version 4.18 (2020-02-17) on a MacBookPro10,2, where I removed the MacOS and installed Ubuntu 20.04.1 LTS/focal.

UPDATE ON THE PREVIOUS COMMENT

It seems that the background isn't being rendered at all.

Here an example without moving to another workspace:

Use $mod+[0-9] to create a new workspace;
Use Shift+$mod+Space to enter into the float-mode.

It also behaves weirdly with the resize mode, and possible with the drag mode.
( @orestisfl Please, let me know if you need more data. Thanks! )

@almir-campos this might be a different issue. Do you run a compositor?