Nav: When entering into a NavFlattened child we only consider the visible items for scoring (note that this only work assuming the NavFlattened child window has interactive items). Fixes accidentally hoping into a NavFlattened child. (#767)

TODO.txt

@@ -253,8 +253,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - nav: allow input system to be be more tolerant of io.DeltaTime=0.0f
  - nav: ESC within a menu of a child window seems to exit the child window.
  - nav: ESC on a flattened child
- - nav: NavFlattened: broken: in typical usage scenario, the items of a fully clipped child wouldn't be considered to enter into a NavFlattened child.
- - nav: NavFlattened: broken: can accidentally enter a NavFlattened child as child items are scored outside of child visible bounds.
+ - nav: NavFlattened: broken: in typical usage scenario, the items of a fully clipped child are currently not considered to enter into a NavFlattened child.
  - nav: NavFlattened: init request doesn't select items that are part of a NavFlattened child
  - nav: Left within a tree node block as a fallback (ImGuiTreeNodeFlags_NavLeftJumpsBackHere by default?)
  - nav: menus: pressing left-right on a vertically clipped menu bar tends to jump to the collapse/close buttons.

imgui.cpp

@@ -2341,6 +2341,15 @@ static bool NavScoreItem(ImGuiNavMoveResult* result, ImRect cand)
     const ImRect& curr = g.NavScoringRectScreen; // Current modified source rect (NB: we've applied Max.x = Min.x in NavUpdate() to inhibit the effect of having varied item width)
     g.NavScoringCount++;
 
+    if (window != g.NavWindow)
+    {
+        // When crossing through a NavFlattened border, we score items on the other windows fully clipped
+        IM_ASSERT((window->Flags | g.NavWindow->Flags) & ImGuiWindowFlags_NavFlattened);
+        if (!window->ClipRect.Contains(cand))
+            return false;
+        cand.ClipWithFull(window->ClipRect); // This allows the scored item to not overlap other candidates in the parent window
+    }
+
     // We perform scoring on items bounding box clipped by the current clipping rectangle on the other axis (clipping on our movement axis would give us equal scores for all clipped items)
     // For example, this ensure that items in one column are not reached when moving vertically from items in another column.
     NavClampRectToVisibleAreaForMoveDir(g.NavMoveClipDir, cand, window->ClipRect);