[SHELL32] Fix focus glitch when hovering separators with submenu open (#7932)

CORE-20124

Since `TB_HITTEST` returns negative numbers for ID for separator menu items [^1],
shell menu assumes mouse moved outside of the menu popup, highlighting the menu
item for currently open submenu.
To fix this behavior, we can detect separators in `CMenuFocusManager::ProcessMouseMove`
and negate the ID returned by `TB_HITTEST` to make it a positive number again,
so the shell menu wouldn't glitch.

[^1]: https://learn.microsoft.com/en-us/windows/win32/controls/tb-hittest

dll/win32/shell32/shellmenu/CMenuFocusManager.cpp

···
         iHitTestResult = SendMessageW(child, TB_HITTEST, 0, (LPARAM) &pt);
         isTracking = entry->mb->_IsTracking();
 
+        if (iHitTestResult < -1)
+        {
+            // TB_HITTEST would return negative numbers for separators
+            iHitTestResult = -iHitTestResult;
+        }
+        else if (iHitTestResult == -1)
+        {
+            // TB_HITTEST would return -1 in two cases:
+            // 1. the mouse is outside the toolbar;
+            // 2. the mouse is over the first item, and that item is a separator.
+            // Confirm the second scenario by checking first item's rect.
+            RECT rc;
+            SendMessageW(child, TB_GETITEMRECT, 1, (LPARAM)&rc);
+            if (PtInRect(&rc, pt))
+                iHitTestResult = 1;
+        }
+
         if (SendMessage(child, WM_USER_ISTRACKEDITEM, iHitTestResult, 0) == S_FALSE)
         {
             // The current tracked item has changed, notify the toolbar