Yesterday, I noticed that Ctrl+Shift+Home did not select all characters between the caret and the beginning of the document, as expected. Likewise for Ctrl+Shift+End. This change applies for all applications.
The Home, End, Ctrl+Home, Ctrl+End, Shift+Home, Shift+End, and Ctrl+Shift+arrow key/key combinations still work normally. This behavior holds for my laptop keyboard, a Bluetooth keyboard, and the onscreen keyboard. The Alt, Ctrl, Shift, and Fn keys work normally, beyond this specific combination. Caps Lock, Fn Lock, Num Lock, and Scroll Lock, likewise.
Oddly, within a remote desktop Ctrl+Shift+Home does still work.
Clearly some configuration has changed, but I have no idea what; certainly I haven't been mucking about in the keyboard settings. Rebooting has not fixed the problem. Keyboard language settings are normal (I'm only using English, and I disabled the keys for switching languages a long time ago). The Windows display language is English, the country is United States, and the keyboard is Qwerty - nothing unusual. I've been using this machine for 18 months, and the last Windows update was two weeks ago, long before this problem. There are no driver updates available.
I can only think of one thing which could possibly be relevant: recently while editing a Word document, it switched to French spelling corrections, somehow; perhaps I hit an odd keyboard combination. I switched Word back to English.
A similar question has been asked before, but none of the fixes there have changed anything. Has anyone else seen this problem?