I've noticed that Word's reaction to things, and its own display, has bugs.
Sometimes the document area goes dark grey. Scrolling a bit resolves the display but usually means your point in the document has not moved. E.g. go to a bookmark or chapter heading. Goes grey. Wait a while. Scroll. And you are where you started.
Sometimes the effective insertion point is a few lines - maybe an inch and a half - too high. Scrolling a few inches up/down usually resolves that.
Sometimes it just becomes unresponsive - usually when doing an automatic save of a big document to OneDrive.
All the above prevent highlighting/selecting anything.
Switching off autosave helps quite a lot.