SUMMARY
On a small display with a very high scaling factor (Read as: 200% on 1080p display), windows are more likely to have its title bar out-of-the-screen-boundary. This often renders the title bar and its window management buttons (Close, Maximize, Minimize) inaccessible.

You can manually move the window with Meta + Mouse drag but it's better for kwin to have window be positioned in a way that title bar is always visible.

Manual out-of-the-screen-boundary window placement by users should still be allowed (Read as: manually Meta + mouse drag to move the window upwards).


STEPS TO REPRODUCE
1. Use Plasma Wayland session
2. Go to System Settings -> Display and Monitor -> Display Configuration
3. Use only one display with a screen resolution of 1920x1080
4. Change your screen scale to 200% (everything on your display is crowded)
5. Open Kontact, click on New Message

OBSERVED RESULT
In step 3, after changing the screen scale factor the title bar of System Settings is already out of the screen boundary, so window management buttons (Close, Maximize, Minimize) are inaccessible with only a mouse.
In step 5, the Kontact's new message window has its title bar out of the boundary.

For System Settings and Kontact you can still drag the empty area of the toolbar to drag to window, but not every application has this similar design (that allows you to drag window by loosely dragging the empty area of the UI) especially third-party ones.

EXPECTED RESULT
In step 3, after changing the screen scale factor, System Settings is re-positioned so title bar is still visible.
In step 5, the Kontact's new message window is positioned (Read as: moved downwards) so title bar is visible.

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION
Small tablet users and visually impaired users that require a larger screen scaling are more likely to run into this issue.

Since GNOME applications tend to use CSD (Client Side Decorations), the use of the word "title bar" might be not precise enough to refer to "the rectangular area with window management buttons that's located at the top of the window" (on GNOME they're header bar instead), but you get the point, the goal is to have window management buttons and an area that allows window dragging to be always accessible to users.

Windows since Windows 7 has this behavior that positions the window to make sure title bar cannot be moved up beyond screen boundary, Plasma Wayland also has this behavior when moving a window that's within screen boundary, but not when it's already beyond screen boundary (which itself is fine). Windows doesn't have the Meta + Mouse drag equivalent so the title bar being out-of-the-screen-boundary on Windows is a more serious accessibility issue than it is on Plasma. That said, it's still convenient to have the title bar of a window readily accessible even when screen scaling factor is huge.