Debug, Metrics: Added "Tools->Item Picker" tool which allow clicking on a widget to break in the debugger within the item code. The tool calls IM_DEBUG_BREAK() which can be redefined in imconfig.h if needed.

docs/CHANGELOG.txt

@@ -61,6 +61,8 @@ Other Changes:
 - Style: Added style.ColorButtonButton (left/right, defaults to ImGuiDir_Right) to move the color button
   of ColorEdit3/ColorEdit4 functions to either side of the inputs.
 - Misc: Added IMGUI_DISABLE_METRICS_WINDOW imconfig.h setting to explicitly compile out ShowMetricsWindow().
+- Debug, Metrics: Added "Tools->Item Picker" tool which allow clicking on a widget to break in the debugger
+  within the item code. The tool calls IM_DEBUG_BREAK() which can be redefined in imconfig.h if needed.
 - ImDrawList: Fixed CloneOutput() helper crashing. (#1860) [@gviot]
 - ImDrawList::ChannelsSplit(), ImDrawListSplitter: Fixed an issue with merging draw commands between
   channel 0 and 1. (#2624)

imconfig.h

@@ -76,6 +76,11 @@
 //typedef void (*MyImDrawCallback)(const ImDrawList* draw_list, const ImDrawCmd* cmd, void* my_renderer_user_data);
 //#define ImDrawCallback MyImDrawCallback
 
+//---- Debug Tools
+// Use 'Metrics->Tools->Item Picker' to pick widgets with the mouse and break into them for easy debugging.
+//#define IM_DEBUG_BREAK  IM_ASSERT(0)
+//#define IM_DEBUG_BREAK  __debugbreak()
+
 //---- Tip: You can add extra functions within the ImGui:: namespace, here or in your own headers files.
 /*
 namespace ImGui

imgui.cpp

@@ -2862,6 +2862,16 @@ void ImGui::SetHoveredID(ImGuiID id)
     g.HoveredIdAllowOverlap = false;
     if (id != 0 && g.HoveredIdPreviousFrame != id)
         g.HoveredIdTimer = g.HoveredIdNotActiveTimer = 0.0f;
+
+    // [DEBUG] Item Picker tool!
+    // We perform the check here because SetHoveredID() is not frequently called (1~ time a frame), making
+    // the cost of this tool near-zero. We would get slightly better call-stack if we made the test in ItemAdd()
+    // but that would incur a slightly higher cost and may require us to hide this feature behind a define.
+    if (id != 0 && id == g.DebugBreakItemId)
+    {
+        IM_DEBUG_BREAK();
+        g.DebugBreakItemId = 0;
+    }
 }
 
 ImGuiID ImGui::GetHoveredID()
@@ -10180,6 +10190,28 @@ void ImGui::ShowMetricsWindow(bool* p_open)
 
     if (ImGui::TreeNode("Tools"))
     {
+        static bool picking_enabled = false;
+        if (ImGui::Button("Item Picker.."))
+            picking_enabled = true;
+        if (picking_enabled)
+        {
+            const ImGuiID hovered_id = g.HoveredIdPreviousFrame;
+            ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
+            if (ImGui::IsKeyPressedMap(ImGuiKey_Escape))
+                picking_enabled = false;
+            if (ImGui::IsMouseClicked(0) && hovered_id)
+            {
+                g.DebugBreakItemId = hovered_id;
+                picking_enabled = false;
+            }
+            ImGui::SetNextWindowBgAlpha(0.5f);
+            ImGui::BeginTooltip();
+            ImGui::Text("HoveredId: 0x%08X", hovered_id);
+            ImGui::Text("Press ESC to abort picking.");
+            ImGui::TextColored(GetStyleColorVec4(hovered_id ? ImGuiCol_Text : ImGuiCol_TextDisabled), "Click to break in debugger!");
+            ImGui::EndTooltip();
+        }
+
         ImGui::Checkbox("Show windows begin order", &show_windows_begin_order);
         ImGui::Checkbox("Show windows rectangles", &show_windows_rects);
         ImGui::SameLine();
@@ -10225,10 +10257,11 @@ void ImGui::ShowMetricsWindow(bool* p_open)
     }
     ImGui::End();
 }
+
 #else
-void ImGui::ShowMetricsWindow(bool*)
-{
-}
+
+void ImGui::ShowMetricsWindow(bool*) { }
+
 #endif
 
 //-----------------------------------------------------------------------------

imgui_internal.h

@@ -401,7 +401,7 @@ enum ImGuiItemStatusFlags_
     ImGuiItemStatusFlags_Deactivated        = 1 << 5    // Only valid if ImGuiItemStatusFlags_HasDeactivated is set.
 
 #ifdef IMGUI_ENABLE_TEST_ENGINE
-    , // [imgui-test only]
+    , // [imgui_tests only]
     ImGuiItemStatusFlags_Openable           = 1 << 10,  //
     ImGuiItemStatusFlags_Opened             = 1 << 11,  //
     ImGuiItemStatusFlags_Checkable          = 1 << 12,  //
@@ -1028,6 +1028,9 @@ struct ImGuiContext
     int                     LogDepthToExpand;
     int                     LogDepthToExpandDefault;            // Default/stored value for LogDepthMaxExpand if not specified in the LogXXX function call.
 
+    // Debug Tools
+    ImGuiID                 DebugBreakItemId;
+
     // Misc
     float                   FramerateSecPerFrame[120];          // Calculate estimate of framerate for user over the last 2 seconds.
     int                     FramerateSecPerFrameIdx;
@@ -1153,6 +1156,8 @@ struct ImGuiContext
         LogDepthRef = 0;
         LogDepthToExpand = LogDepthToExpandDefault = 2;
 
+        DebugBreakItemId = 0;
+
         memset(FramerateSecPerFrame, 0, sizeof(FramerateSecPerFrame));
         FramerateSecPerFrameIdx = 0;
         FramerateSecPerFrameAccum = 0.0f;
@@ -1662,7 +1667,19 @@ IMGUI_API void              ImFontAtlasBuildFinish(ImFontAtlas* atlas);
 IMGUI_API void              ImFontAtlasBuildMultiplyCalcLookupTable(unsigned char out_table[256], float in_multiply_factor);
 IMGUI_API void              ImFontAtlasBuildMultiplyRectAlpha8(const unsigned char table[256], unsigned char* pixels, int x, int y, int w, int h, int stride);
 
-// Test engine hooks (imgui-test)
+// Debug Tools
+// Use 'Metrics->Tools->Item Picker' to break into the call-stack of a specific item.
+#ifndef IM_DEBUG_BREAK      
+#if defined(__clang__)
+#define IM_DEBUG_BREAK()    __builtin_debugtrap()
+#elif defined (_MSC_VER)
+#define IM_DEBUG_BREAK()    __debugbreak()
+#else
+#define IM_DEBUG_BREAK()    IM_ASSERT(0)    // It is expected that you define IM_DEBUG_BREAK() into something that will break nicely in a debugger!
+#endif
+#endif // #ifndef IM_DEBUG_BREAK
+
+// Test Engine Hooks (imgui_tests)
 //#define IMGUI_ENABLE_TEST_ENGINE
 #ifdef IMGUI_ENABLE_TEST_ENGINE
 extern void                 ImGuiTestEngineHook_PreNewFrame(ImGuiContext* ctx);