Windows: BeginChild(): made it possible to call SetNextWindowSize() on a child window using ImGuiChildFlags_ResizeX/ImGuiChildFlags_ResizeY. (#1710, #8020)

docs/CHANGELOG.txt

@@ -43,6 +43,9 @@ Breaking changes:
 
 Other changes:
 
+- Windows: BeginChild(): made it possible to call SetNextWindowSize() on a child window
+  using ImGuiChildFlags_ResizeX,ImGuiChildFlags_ResizeY in order to override its current
+  size. (#1710, #8020)
 - Scrollbar: Shift+Click scroll to clicked location (pre-1.90.8 default). (#8002, #7328)
 - Scrollbar: added io.ConfigScrollbarScrollByPage setting (default to true). (#8002, #7328)
   Set io.ConfigScrollbarScrollByPage=false to enforce always scrolling to clicked location.

imgui.cpp

@@ -5917,18 +5917,35 @@ bool ImGui::BeginChildEx(const char* name, ImGuiID id, const ImVec2& size_arg, I
         window_flags |= ImGuiWindowFlags_NoMove;
     }
 
-    // Forward child flags
-    g.NextWindowData.Flags |= ImGuiNextWindowDataFlags_HasChildFlags;
-    g.NextWindowData.ChildFlags = child_flags;
-
     // Forward size
     // Important: Begin() has special processing to switch condition to ImGuiCond_FirstUseEver for a given axis when ImGuiChildFlags_ResizeXXX is set.
     // (the alternative would to store conditional flags per axis, which is possible but more code)
     const ImVec2 size_avail = GetContentRegionAvail();
     const ImVec2 size_default((child_flags & ImGuiChildFlags_AutoResizeX) ? 0.0f : size_avail.x, (child_flags & ImGuiChildFlags_AutoResizeY) ? 0.0f : size_avail.y);
-    const ImVec2 size = CalcItemSize(size_arg, size_default.x, size_default.y);
+    ImVec2 size = CalcItemSize(size_arg, size_default.x, size_default.y);
+
+    // A SetNextWindowSize() call always has priority (#8020)
+    // (since the code in Begin() never supported SizeVal==0.0f aka auto-resize via SetNextWindowSize() call, we don't support it here for now)
+    // FIXME: We only support ImGuiCond_Always in this path. Supporting other paths would requires to obtain window pointer.
+    if ((g.NextWindowData.Flags & ImGuiNextWindowDataFlags_HasSize) != 0 && (g.NextWindowData.SizeCond & ImGuiCond_Always) != 0)
+    {
+        if (g.NextWindowData.SizeVal.x > 0.0f)
+        {
+            size.x = g.NextWindowData.SizeVal.x;
+            child_flags &= ~ImGuiChildFlags_ResizeX;
+        }
+        if (g.NextWindowData.SizeVal.y > 0.0f)
+        {
+            size.y = g.NextWindowData.SizeVal.y;
+            child_flags &= ~ImGuiChildFlags_ResizeY;
+        }
+    }
     SetNextWindowSize(size);
 
+    // Forward child flags
+    g.NextWindowData.Flags |= ImGuiNextWindowDataFlags_HasChildFlags;
+    g.NextWindowData.ChildFlags = child_flags;
+
     // Build up name. If you need to append to a same child from multiple location in the ID stack, use BeginChild(ImGuiID id) with a stable value.
     // FIXME: 2023/11/14: commented out shorted version. We had an issue with multiple ### in child window path names, which the trailing hash helped workaround.
     // e.g. "ParentName###ParentIdentifier/ChildName###ChildIdentifier" would get hashed incorrectly by ImHashStr(), trailing _%08X somehow fixes it.

imgui.h

@@ -29,7 +29,7 @@
 // Library Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals, e.g. '#if IMGUI_VERSION_NUM >= 12345')
 #define IMGUI_VERSION       "1.91.3 WIP"
-#define IMGUI_VERSION_NUM   19121
+#define IMGUI_VERSION_NUM   19122
 #define IMGUI_HAS_TABLE
 
 /*

imgui_demo.cpp

@@ -3931,6 +3931,9 @@ static void ShowDemoWindowLayout()
         ImGui::SeparatorText("Manual-resize");
         {
             HelpMarker("Drag bottom border to resize. Double-click bottom border to auto-fit to vertical contents.");
+            //if (ImGui::Button("Set Height to 200"))
+            //    ImGui::SetNextWindowSize(ImVec2(-FLT_MIN, 200.0f));
+
             ImGui::PushStyleColor(ImGuiCol_ChildBg, ImGui::GetStyleColorVec4(ImGuiCol_FrameBg));
             if (ImGui::BeginChild("ResizableChild", ImVec2(-FLT_MIN, ImGui::GetTextLineHeightWithSpacing() * 8), ImGuiChildFlags_Borders | ImGuiChildFlags_ResizeY))
                 for (int n = 0; n < 10; n++)