When I resize freshly opened Skype window (either maximized from the tray icon or just new instance of skype), it only resizes by several pixels (it resizes the window twice) and then freezes. After randomly moving/resizing the window several times (didn't manage to find regularity), resize starts working well. Apparently, this is an issue with Qt4 applications: I've managed to reproduce it only with Skype and FBReader. It is also an issue with Wolfram Mathematica; however, not sure which toolkit they use. I've also tested Qt5 applications with both wayland and xcb platforms (Dolphin), GTK applications (Inkscape), some Java applications (SweetHome3D) -- all of them work fine.
Unfortunately I don't have any Qt 4 applications around any more to test.
Just found one by accident: kdebugdialog and unfortunately I must confirm the behavior you describe.
Resize xwayland apps (qt4 and gtk3) is not smooth on neon dev unstable. And I see black borders similar to that https://bug767212.bugzilla-attachments.gnome.org/attachment.cgi?id=329082 It's more evident when I resize vlc player window.
I figured out why this happens: the timestamp handling only works on X11, thus the sync request has an outdated timestamp.
Workaround at: https://phabricator.kde.org/D7524
Git commit 6f4c02ce10186fe966bce1ead63144e845bc74fe by Martin Flöser. Committed on 25/08/2017 at 13:49. Pushed by graesslin into branch 'master'. Disable synced resizing for X11 clients on Xwayland Summary: The sync is broken due to updateXTime not working correctly on Wayland. As the broken sync results in a very unsmooth resize experience it is better to disable it for the time being. Once we have a solution for updateXTime it should be enabled again. Test Plan: Run nested kwin_wayland and resized Qt 4 application Reviewers: #kwin, #plasma Subscribers: plasma-devel, kwin Tags: #kwin Differential Revision: https://phabricator.kde.org/D7524 M +3 -1 client.cpp https://commits.kde.org/kwin/6f4c02ce10186fe966bce1ead63144e845bc74fe