restore window focus after lock screen

src/Wm.zig

@@ -449,6 +449,7 @@ fn onWmEvent(_: *river.WindowManagerV1, event: river.WindowManagerV1.Event, self
         },
         .session_unlocked => {
             self.locked = false;
+            for (self.seats.items) |seat| self.restoreFocus(seat);
             self.ipcDirty();
         },
 
@@ -811,6 +812,25 @@ fn validateFocus(self: *Wm, seat: *Seat) void {
     }
 }
 
+/// Put the keyboard back where it was before the session locked.
+///
+/// While locked the lock surface holds focus and river drops it again on
+/// unlock, but our own `focused` never changed — nothing looks stale, so
+/// `validateFocus` sees no work and no focus request is ever made. The seat
+/// then sits there with no window focused until the user clicks one. Re-issue
+/// the focus, falling back the way `validateFocus` does if that window went
+/// away while the screen was locked.
+fn restoreFocus(self: *Wm, seat: *Seat) void {
+    if (seat.focused) |win| {
+        if (!win.closed and win.visible) {
+            seat.focus(win);
+            return;
+        }
+    }
+    const out = seat.currentOutput() orelse return;
+    if (self.topVisible(out)) |win| seat.focus(win);
+}
+
 /// View a set of tags, on whichever screen owns them.
 ///
 /// With the tag set split across displays, `Mod+7` is as much "go to the screen