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/InputManager.hpp

@@ -184,6 +184,8 @@ class CInputManager {
     // for some bugs in follow mouse 0
     bool m_bLastFocusOnLS = false;
 
+    bool m_bLastFocusOnIMEPopup = false;
+
     // for hiding cursor on touch
     bool m_bLastInputTouch = false;