When you resize a window you generally are doing so to get the contents to fit better, but by default, at least on Mint KDE, the window becomes transparent working against this goal. Expectation: while the option could be there for those who want it, by default it should be off. Reproducible: Always
Could also be a bug (didn't check the video). You are running inside a VM, right? Please attach the output of Konsole command "qdbus org.kde.kwin /KWin supportInformation".
All I get is: qdbus: could not find a Qt installation of '' Installed Qt5-default, then got: qdbus: could not exec '/usr/lib/x86_64-linux-gnu/qt5/bin/qdbus': No such file or directory
kcmshell4 kwincompositing, 2nd tab, find the translucency effect, press the config button, uncheck transparency for resizing. result?
System Settings, Desktop Effects, All Effects Tab, Translucency, Moving Windows (which also effects resizing windows) Yes, that works - but the comment here is that while it is fine to have such a setting, given how it is something which obfuscates data you are focusing on the default should be to be fully opaque. At the very least the setting should be trivial to find - most users will never dig down to it (most users never change any significant settings).
As it's not a bug, but intended behavior I change to wishlist to reflect this. Now the idea is to make it easier to resize the window. While the user resizes this operation is more important than the content, thus we want to support it. During resize it might matter where other windows are and thus the translucency helps. This is the intended workflow. I'm sorry that our default doesn't match your expectations and your workflow, but it's unfortunately impossible to find defaults which suit all users.
Adding it to the wish list makes more sense. Thank you.