The document switcher plugin defines its default shortcuts as Ctrl+Tab and Ctrl+Shift+Tab. On Mac this translates to Command[+Shift]+Tab, which are intercepted by the host and trigger the application switcher. I *think* that something similar will happen on MSWindows.
To make things more complicated, Qt/Mac has an attribute that tells the keyboard event functions NOT to swap the Control and Meta (Command) keys, and an as-yet unidentified bug (probably) in KF5 code causes Control+Tab shortcuts to be discarded.
The patch introduces a platform-specific shortcutAccelerator variable that is set to Qt::CTRL except when running on Mac and the DontSwapCtrlAndMeta attribute is set. In the latter case it is set to Qt::ALT (instead of Qt::META). If the Qt::ALT modifier is defined on MSWindows it could be used there too.