Multi-Select: Box-Select: fixes for using accross nested child windows. (#8364)

- IsFocused scan nav focus route.
- When covering multiple windows, draw in front most ones (grabbed FindFrontMostVisibleChildWindow() from docking branch).

docs/CHANGELOG.txt

@@ -78,6 +78,7 @@ Other Changes:
     and ImGuiSelectionUserData is technically opaque storage. (#7994, #1861)
     (we will probably bring this back as a minor optimization if we have a way to for
     user to tell us ImGuiSelectionUserData are indices)
+  - Box-Select: fixes for using accross nested child windows. (#8364)
   - Box-Select + Clipper: fixed an issue selecting items while scrolling while a clipper
     active. (#7994, #8250, #7821, #7850, #7970)
   - Box-Select + Tables: fixed an issue when calling `BeginMultiSelect()` in a table

imgui.cpp

@@ -6588,6 +6588,14 @@ void ImGui::EndChild()
     g.LogLinePosY = -FLT_MAX; // To enforce a carriage return
 }
 
+ImGuiWindow* ImGui::FindFrontMostVisibleChildWindow(ImGuiWindow* window)
+{
+    for (int n = window->DC.ChildWindows.Size - 1; n >= 0; n--)
+        if (IsWindowActiveAndVisible(window->DC.ChildWindows[n]))
+            return FindFrontMostVisibleChildWindow(window->DC.ChildWindows[n]);
+    return window;
+}
+
 static void SetWindowConditionAllowFlags(ImGuiWindow* window, ImGuiCond flags, bool enabled)
 {
     window->SetWindowPosAllowFlags       = enabled ? (window->SetWindowPosAllowFlags       | flags) : (window->SetWindowPosAllowFlags       & ~flags);
@@ -8754,6 +8762,17 @@ void ImGui::PopFocusScope()
     g.CurrentFocusScopeId = g.FocusScopeStack.Size ? g.FocusScopeStack.back().ID : 0;
 }
 
+bool ImGui::IsInNavFocusRoute(ImGuiID focus_scope_id)
+{
+    ImGuiContext& g = *GImGui;
+    if (g.NavFocusScopeId == focus_scope_id)
+        return true;
+    for (const ImGuiFocusScopeData& focus_scope : g.NavFocusRoute)
+        if (focus_scope.ID == focus_scope_id)
+            return true;
+    return false;
+}
+
 void ImGui::SetNavFocusScope(ImGuiID focus_scope_id)
 {
     ImGuiContext& g = *GImGui;

imgui_internal.h

@@ -3328,6 +3328,7 @@ namespace ImGui
 
     // Childs
     IMGUI_API bool          BeginChildEx(const char* name, ImGuiID id, const ImVec2& size_arg, ImGuiChildFlags child_flags, ImGuiWindowFlags window_flags);
+    IMGUI_API ImGuiWindow*  FindFrontMostVisibleChildWindow(ImGuiWindow* window);
 
     // Popups, Modals
     IMGUI_API bool          BeginPopupEx(ImGuiID id, ImGuiWindowFlags extra_window_flags);
@@ -3481,6 +3482,7 @@ namespace ImGui
     // We don't use the ID Stack for this as it is common to want them separate.
     IMGUI_API void          PushFocusScope(ImGuiID id);
     IMGUI_API void          PopFocusScope();
+    IMGUI_API bool          IsInNavFocusRoute(ImGuiID focus_scope_id);
     inline ImGuiID          GetCurrentFocusScope() { ImGuiContext& g = *GImGui; return g.CurrentFocusScopeId; }   // Focus scope we are outputting into, set by PushFocusScope()
 
     // Drag and Drop

imgui_widgets.cpp

@@ -7914,8 +7914,9 @@ void ImGui::EndBoxSelect(const ImRect& scope_rect, ImGuiMultiSelectFlags ms_flag
     bs->EndPosRel = WindowPosAbsToRel(window, ImClamp(g.IO.MousePos, scope_rect.Min, scope_rect.Max)); // Clamp stored position according to current scrolling view
     ImRect box_select_r = bs->BoxSelectRectCurr;
     box_select_r.ClipWith(scope_rect);
-    window->DrawList->AddRectFilled(box_select_r.Min, box_select_r.Max, GetColorU32(ImGuiCol_SeparatorHovered, 0.30f)); // FIXME-MULTISELECT: Styling
-    window->DrawList->AddRect(box_select_r.Min, box_select_r.Max, GetColorU32(ImGuiCol_NavCursor)); // FIXME-MULTISELECT FIXME-DPI: Styling
+    ImGuiWindow* draw_window = FindFrontMostVisibleChildWindow(window);
+    draw_window->DrawList->AddRectFilled(box_select_r.Min, box_select_r.Max, GetColorU32(ImGuiCol_SeparatorHovered, 0.30f)); // FIXME-MULTISELECT: Styling
+    draw_window->DrawList->AddRect(box_select_r.Min, box_select_r.Max, GetColorU32(ImGuiCol_NavCursor)); // FIXME-MULTISELECT FIXME-DPI: Styling
 
     // Scroll
     const bool enable_scroll = (ms_flags & ImGuiMultiSelectFlags_ScopeWindow) && (ms_flags & ImGuiMultiSelectFlags_BoxSelectNoScroll) == 0;
@@ -8014,10 +8015,10 @@ ImGuiMultiSelectIO* ImGui::BeginMultiSelect(ImGuiMultiSelectFlags flags, int sel
     ms->Clear();
     ms->FocusScopeId = id;
     ms->Flags = flags;
-    ms->IsFocused = (ms->FocusScopeId == g.NavFocusScopeId);
     ms->BackupCursorMaxPos = window->DC.CursorMaxPos;
     ms->ScopeRectMin = window->DC.CursorMaxPos = window->DC.CursorPos; // CalcScopeRect() for ImGuiMultiSelectFlags_ScopeRect will measure in EndMultiSelect().
     PushFocusScope(ms->FocusScopeId);
+    ms->IsFocused = IsInNavFocusRoute(g.CurrentFocusScopeId);
     if (flags & ImGuiMultiSelectFlags_ScopeWindow) // Mark parent child window as navigable into, with highlight. Assume user will always submit interactive items.
         window->DC.NavLayersActiveMask |= 1 << ImGuiNavLayer_Main;
 
@@ -8145,7 +8146,7 @@ ImGuiMultiSelectIO* ImGui::EndMultiSelect()
     // Clear selection when clicking void?
     // We specifically test for IsMouseDragPastThreshold(0) == false to allow box-selection!
     // The InnerRect test is necessary for non-child/decorated windows.
-    bool scope_hovered = IsWindowHovered() && window->InnerRect.Contains(g.IO.MousePos);
+    bool scope_hovered = window->InnerRect.Contains(g.IO.MousePos) && IsWindowHovered(ImGuiHoveredFlags_ChildWindows);
     if (scope_hovered && (ms->Flags & ImGuiMultiSelectFlags_ScopeRect))
         scope_hovered &= scope_rect.Contains(g.IO.MousePos);
     if (scope_hovered && g.HoveredId == 0 && g.ActiveId == 0)