Comments by a developer inside the Windows Media Player source code see https://pastebin.com/PTLeWhc2

mplayer.c

[...]
/* RouteKeyPresses
 *
 * Reroutes cursor keys etc to track bar.
 */
void RouteKeyPresses(PMSG pMsg)
{
    /* Hack for PowerPoint
     *
     * Mail from PaulWa:
     *
     * --------
     * Here's a problem you might consider fixing.
     * Launching Media Player with certain keystrokes
     * doesn't work right (e.g. arrow keys, page up/down,
     * etc.).
     *
     * The problem is due to the fact that Media Player
     * handles key up events.  We use the key down event
     * to launch the server in slideshow, but then the key
     * up event is passed to the server.  It would probably
     * be best for Media Player to ignore key up events
     * unless it had previously received a key down.
     * If this is very difficult to fix in Media Player,
     * then we can fix it in PP by launching servers on
     * key up rather than key down.  However, other container
     * apps will see the same problem.
     * --------
     *
     * OK, in the spirit of cooperation, let's hack things
     * so our PowerPoint friends can carry on with their
     * dubious practices.
     */
    static WPARAM LastVKeyDown;
 
[...]
    if (rMsg.message == WM_TIMER && rMsg.hwnd == NULL) {
#ifdef CHICAGO_PRODUCT
        /* The reason for requiring the following test is now lost
         * in the mists of time.  Now this app is 32-bit, these
         * bogus timer callbacks (if they really do still occur)
         * could be 16-bit, so we need to add yet more ugliness
         * in the form of assembler to an app which is already
         * hardly a paragon of pulchritude.
         *
         * A plea:
         *
         * If you add some obscure code such as below, to this or
         * any other app, even if it has only the teeniest chance
         * of being less blindingly obvious to someone else than
         * it is to you at the time of writing, please please please
         * add a f***ing comment.
         *
         * Respectfully,
         * A Developer
         */