Just for clarification: you mean maximization and not fullscreen, right? That is click the maximize button and not using a fullscreen menu item.

Created attachment 100861 [details]
attachment-7947-0.html

I do not maximize or set fullscreen, the initial geometry of the window is
too large and overlaps under the Plasma panel.
Maybe the geometry of the maximized window was stored from a session
without a panel, persists and is not clipped by KWin/Wayland.

On Wed, Aug 31, 2016 at 3:01 PM, Martin Gräßlin via KDE Bugzilla <
bugzilla_noreply@kde.org> wrote:

> https://bugs.kde.org/show_bug.cgi?id=366632
>
> --- Comment #1 from Martin Gräßlin <mgraesslin@kde.org> ---
> Just for clarification: you mean maximization and not fullscreen, right?
> That
> is click the maximize button and not using a fullscreen menu item.
>
> --
> You are receiving this mail because:
> You reported the bug.

Thanks for clarifying. Now I understand what you mean and yes, I have seen this as well. Not really sure how we can address it yet, but It's an important issue.

Git commit e1bcda92a2f446ecfa4538e28bcc697344cef8f5 by Martin Gräßlin.
Committed on 14/09/2016 at 12:08.
Pushed by graesslin into branch 'master'.

[autotests/integration] Test case for window larger than screen

If a window gets opened which will be too large with decorations it
should get a configure event with a smaller size. This currently
doesn't happen as the test highlights.

M  +38   -0    autotests/integration/shell_client_test.cpp

http://commits.kde.org/kwin/e1bcda92a2f446ecfa4538e28bcc697344cef8f5

I can confirm this behaviour:
* every application previously closed in maximized window state will reopen with this problem;
* applications for which I have set a window rule to "initially maximize" don't have this behaviour, they simply appear correctly maximized.

(The second point is tested only with Firefox for which I have set this rule long time ago, I am unable to set a similar rule on other applications on Wayland, but I have tried only few minutes.)

same on Arch Linux under Wayland.
When I open desktop settings window, buttons on botton are hidden behind plasma panel.

(In reply to Dr. Chapatin from comment #6)
> same on Arch Linux under Wayland.
> When I open desktop settings window, buttons on botton are hidden behind
> plasma panel.

Can reproduce

Distribution: KDE neon Developer Edition
Kernel: 4.15.0-29-generic
Plasma: 5.13.80
Apps: 18.11.70
Qt: 5.11.1
Frameworks: 5.48.0
Graphics:
Card-1: Intel driver: i915 v: kernel bus ID: 00:02.0 chip ID: 8086:5917 
Card-2: NVIDIA GM108M [GeForce 940MX] driver: nouveau v: kernel bus ID: 01:00.0 chip ID: 10de:134d 
Display: wayland server: X.Org 1.19.6 driver: modesetting,nouveau 
unloaded: fbdev,vesa alternate: nvidia compositor: kwin wayland 
resolution: 1920x1080~57Hz 
OpenGL: renderer: Mesa DRI Intel UHD Graphics 620 (Kabylake GT2) v: 4.5 Mesa 18.0.5 
compat-v: 3.0 direct render: Yes

*** Bug 398806 has been marked as a duplicate of this bug. ***

This bug persists on neon dev unstable.

plasma 5.16 beta has the same bug :(

Operating System: Arch Linux 
KDE Plasma Version: 5.15.90
KDE Frameworks Version: 5.58.0
Qt Version: 5.13.0 beta3

I can't reproduce this bug on Arch Linux.
If I'm not mistaken, David Edmundson recently fixed a Qt issue that caused windows to increase in size each time they opened.

Operating System: Arch Linux 
KDE Plasma Version: 5.16.4
KDE Frameworks Version: 5.60.0
Qt Version: 5.13.0

This bug persists on neon unstable. When I open a 1080p video with Haruna player
on my laptop with 1366x768 screen, the seek bar on bottom of Haruna is hidden
behind Plasma panel. Also the buttons on bottom of the drkonqi helper to install
debug packages are hidden behind Plasma panel.

Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.22.80
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.3
Graphics Platform: Wayland

Fixed 
https://invent.kde.org/plasma/kwin/-/merge_requests/1423

Nice!