IME: fix IME popup mouse inputs (again) (#5417)

`lastBoxLocal`'s size should be the actual popup's size instead of the cursor
rectangle's size. Also, the rectangle position is now relative to the popup.
(Actually fixes #5255 imho.)

One thing #3922 missed was handling focus held by buttons. Let's hope I get
it right this time.

src/managers/input/InputMethodPopup.hpp

@@ -32,11 +32,10 @@ class CInputPopup {
     wlr_input_popup_surface_v2* pWlr = nullptr;
     CWLSurface                  surface;
     CBox                        lastBoxLocal;
-    Vector2D                    lastPopupSize;
     uint64_t                    lastMonitor = -1;
 
     DYNLISTENER(mapPopup);
     DYNLISTENER(unmapPopup);
     DYNLISTENER(destroyPopup);
     DYNLISTENER(commitPopup);
-};
+};