SUMMARY

Key events are sent to applications when moving windows with the keyboard.  This behaviour is clearly unintended.  For example when moving a window of a text editor, the cursor is moved in the text when pressing the arrow keys.

STEPS TO REPRODUCE
1. Open any application the arrow keys and ENTER are handled by the application. (For example: Kate with some text or code)
2. ALT-F3 -> ALT-M -> ENTER
3. Use the arrow keys move the window

OBSERVED RESULT
The window moves, but the application also reacts to the key events

EXPECTED RESULT
The window moves and nothing is changed in the application (it does not receive key events until the move is completed by hitting ENTER or clicking with the mouse)

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.13.3
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION
I also noticed that the behaviour is not exactly the same when trying to move non-KDE applications such as Firefox: The arrow keys do not move the window and ENTER does not complete the move operation.  The only way to complete the move operation is to left-click with the mouse.  The key events are handled by the application as if no move operation had been started.