Windows: BeginChild(): Fixed auto-resizing erroneously limiting size to host viewport minus padding. (#7063)

docs/CHANGELOG.txt

@@ -44,6 +44,8 @@ Breaking changes:
 
 Other changes:
 
+- Windows: BeginChild(): Fixed auto-resizing erroneously limiting size to host viewport
+  minus padding. There are no limit to a child width/height. (#7063) [@Devyre]
 - Drag and Drop: Fixed drop target highlight on items temporarily pushing a widened clip rect
   (namely Selectables and Treenodes using SpanAllColumn flag) so the highlight properly covers
   all columns. (#7049, #4281, #3272)

imgui.cpp

@@ -5788,8 +5788,8 @@ static ImVec2 CalcWindowAutoFitSize(ImGuiWindow* window, const ImVec2& size_cont
     {
         // Maximum window size is determined by the viewport size or monitor size
         ImVec2 size_min = CalcWindowMinSize(window);
-        ImVec2 avail_size = ImGui::GetMainViewport()->WorkSize;
-        ImVec2 size_auto_fit = ImClamp(size_desired, size_min, ImMax(size_min, avail_size - style.DisplaySafeAreaPadding * 2.0f));
+        ImVec2 size_max = (window->Flags & ImGuiWindowFlags_ChildWindow) ? ImVec2(FLT_MAX, FLT_MAX) : ImGui::GetMainViewport()->WorkSize - style.DisplaySafeAreaPadding * 2.0f;
+        ImVec2 size_auto_fit = ImClamp(size_desired, size_min, size_max);
 
         // When the window cannot fit all contents (either because of constraints, either because screen is too small),
         // we are growing the size on the other axis to compensate for expected scrollbar. FIXME: Might turn bigger than ViewportSize-WindowPadding.