Added a mechanism to compact/free the larger allocations of unused windows (buffers are compacted when a window is unused for 60 seconds, as per io.ConfigWindowsMemoryCompactTimer = 60.0f). Note that memory usage has never been reported as a problem, so this is merely a touch of overzealous luxury. (#2636)

docs/CHANGELOG.txt

@@ -42,6 +42,9 @@ Other Changes:
 - SliderScalar: Improved assert when using U32 or U64 types with a large v_max value. (#2765) [@loicmouton]
 - ImDrawList: clarified the name of many parameters so reading the code is a little easier. (#2740)
 - Using offsetof() when available in C++11. Avoids Clang sanitizer complaining about old-style macros. (#94)
+- Added a mechanism to compact/free the larger allocations of unused windows (buffers are compacted when
+  a window is unused for 60 seconds, as per io.ConfigWindowsMemoryCompactTimer = 60.0f). Note that memory
+  usage has never been reported as a problem, so this is merely a touch of overzealous luxury. (#2636)
 - Backends: DX11: Fixed GSGetShader() call not passing an initialized instance count,
   would generally make the debug layer complain (Added in 1.72).
 - Backends: Vulkan: Added support for specifying multisample count.

imgui.cpp

@@ -1244,6 +1244,7 @@ ImGuiIO::ImGuiIO()
     ConfigInputTextCursorBlink = true;
     ConfigWindowsResizeFromEdges = true;
     ConfigWindowsMoveFromTitleBarOnly = false;
+    ConfigWindowsMemoryCompactTimer = 60.0f;
 
     // Platform Functions
     BackendPlatformName = BackendRendererName = NULL;
@@ -2699,6 +2700,7 @@ ImGuiWindow::ImGuiWindow(ImGuiContext* context, const char* name)
     SetWindowPosVal = SetWindowPosPivot = ImVec2(FLT_MAX, FLT_MAX);
 
     LastFrameActive = -1;
+    LastTimeActive = -1.0f;
     ItemWidthDefault = 0.0f;
     FontWindowScale = 1.0f;
     SettingsIdx = -1;
@@ -2713,6 +2715,9 @@ ImGuiWindow::ImGuiWindow(ImGuiContext* context, const char* name)
     NavLastIds[0] = NavLastIds[1] = 0;
     NavRectRel[0] = NavRectRel[1] = ImRect();
     NavLastChildNavWindow = NULL;
+
+    MemoryCompacted = false;
+    MemoryDrawListIdxCapacity = MemoryDrawListVtxCapacity = 0;
 }
 
 ImGuiWindow::~ImGuiWindow()
@@ -2783,6 +2788,36 @@ static void SetCurrentWindow(ImGuiWindow* window)
         g.FontSize = g.DrawListSharedData.FontSize = window->CalcFontSize();
 }
 
+// Free up/compact internal window buffers, we can use this when a window becomes unused.
+// This is currently unused by the library, but you may call this yourself for easy GC.
+// Not freed:
+// - ImGuiWindow, ImGuiWindowSettings, Name
+// - StateStorage, ColumnsStorage (may hold useful data)
+// This should have no noticeable visual effect. When the window reappear however, expect new allocation/buffer growth/copy cost.
+void ImGui::GcCompactTransientWindowBuffers(ImGuiWindow* window)
+{
+    window->MemoryCompacted = true;
+    window->MemoryDrawListIdxCapacity = window->DrawList->IdxBuffer.Capacity;
+    window->MemoryDrawListVtxCapacity = window->DrawList->VtxBuffer.Capacity;
+    window->IDStack.clear();
+    window->DrawList->ClearFreeMemory();
+    window->DC.ChildWindows.clear();
+    window->DC.ItemFlagsStack.clear();
+    window->DC.ItemWidthStack.clear();
+    window->DC.TextWrapPosStack.clear();
+    window->DC.GroupStack.clear();
+}
+
+void ImGui::GcAwakeTransientWindowBuffers(ImGuiWindow* window)
+{
+    // We stored capacity of the ImDrawList buffer to reduce growth-caused allocation/copy when awakening.
+    // The other buffers tends to amortize much faster.
+    window->MemoryCompacted = false;
+    window->DrawList->IdxBuffer.reserve(window->MemoryDrawListIdxCapacity);
+    window->DrawList->VtxBuffer.reserve(window->MemoryDrawListVtxCapacity);
+    window->MemoryDrawListIdxCapacity = window->MemoryDrawListVtxCapacity = 0;
+}
+
 void ImGui::SetNavID(ImGuiID id, int nav_layer)
 {
     ImGuiContext& g = *GImGui;
@@ -3814,8 +3849,9 @@ void ImGui::NewFrame()
 
     g.NavIdTabCounter = INT_MAX;
 
-    // Mark all windows as not visible
+    // Mark all windows as not visible and compact unused memory.
     IM_ASSERT(g.WindowsFocusOrder.Size == g.Windows.Size);
+    const float memory_compact_start_time = (g.IO.ConfigWindowsMemoryCompactTimer > 0.0f) ? (float)g.Time - g.IO.ConfigWindowsMemoryCompactTimer : FLT_MAX;
     for (int i = 0; i != g.Windows.Size; i++)
     {
         ImGuiWindow* window = g.Windows[i];
@@ -3823,6 +3859,10 @@ void ImGui::NewFrame()
         window->BeginCount = 0;
         window->Active = false;
         window->WriteAccessed = false;
+
+        // Garbage collect (this is totally functional but we may need decide if the side-effects are desirable)
+        if (!window->WasActive && !window->MemoryCompacted && window->LastTimeActive < memory_compact_start_time)
+            GcCompactTransientWindowBuffers(window);
     }
 
     // Closing the focused window restore focus to the first active root window in descending z-order
@@ -5391,6 +5431,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     {
         window->Flags = (ImGuiWindowFlags)flags;
         window->LastFrameActive = current_frame;
+        window->LastTimeActive = (float)g.Time;
         window->BeginOrderWithinParent = 0;
         window->BeginOrderWithinContext = (short)(g.WindowsActiveCount++);
     }
@@ -5404,6 +5445,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     ImGuiWindow* parent_window = first_begin_of_the_frame ? ((flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_Popup)) ? parent_window_in_stack : NULL) : window->ParentWindow;
     IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));
 
+    // We allow window memory to be compacted so recreate the base stack when needed.
+    if (window->IDStack.Size == 0)
+        window->IDStack.push_back(window->ID);
+
     // Add to stack
     // We intentionally set g.CurrentWindow to NULL to prevent usage until when the viewport is set, then will call SetCurrentWindow()
     g.CurrentWindowStack.push_back(window);
@@ -5468,6 +5513,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->ClipRect = ImVec4(-FLT_MAX,-FLT_MAX,+FLT_MAX,+FLT_MAX);
         window->IDStack.resize(1);
 
+        // Restore buffer capacity when woken from a compacted state, to avoid
+        if (window->MemoryCompacted)
+            GcAwakeTransientWindowBuffers(window);
+
         // Update stored window name when it changes (which can _only_ happen with the "###" operator, so the ID would stay unchanged).
         // The title bar always display the 'name' parameter, so we only update the string storage if it needs to be visible to the end-user elsewhere.
         bool window_title_visible_elsewhere = false;

imgui.h

@@ -1360,6 +1360,7 @@ struct ImGuiIO
     bool        ConfigInputTextCursorBlink;     // = true           // Set to false to disable blinking cursor, for users who consider it distracting. (was called: io.OptCursorBlink prior to 1.63)
     bool        ConfigWindowsResizeFromEdges;   // = true           // Enable resizing of windows from their edges and from the lower-left corner. This requires (io.BackendFlags & ImGuiBackendFlags_HasMouseCursors) because it needs mouse cursor feedback. (This used to be a per-window ImGuiWindowFlags_ResizeFromAnySide flag)
     bool        ConfigWindowsMoveFromTitleBarOnly; // = false       // [BETA] Set to true to only allow moving windows when clicked+dragged from the title bar. Windows without a title bar are not affected.
+    float       ConfigWindowsMemoryCompactTimer;// = 60.0f          // [BETA] Compact window memory usage when unused. Set to 0.0f to disable.
 
     //------------------------------------------------------------------
     // Platform Functions

imgui_demo.cpp

@@ -3049,6 +3049,7 @@ void ImGui::ShowAboutWindow(bool* p_open)
         if (io.ConfigInputTextCursorBlink)                              ImGui::Text("io.ConfigInputTextCursorBlink");
         if (io.ConfigWindowsResizeFromEdges)                            ImGui::Text("io.ConfigWindowsResizeFromEdges");
         if (io.ConfigWindowsMoveFromTitleBarOnly)                       ImGui::Text("io.ConfigWindowsMoveFromTitleBarOnly");
+        if (io.ConfigWindowsMemoryCompactTimer > 0.0f)                  ImGui::Text("io.ConfigWindowsMemoryCompactTimer = %.1ff", io.ConfigWindowsMemoryCompactTimer);
         ImGui::Text("io.BackendFlags: 0x%08X", io.BackendFlags);
         if (io.BackendFlags & ImGuiBackendFlags_HasGamepad)             ImGui::Text(" HasGamepad");
         if (io.BackendFlags & ImGuiBackendFlags_HasMouseCursors)        ImGui::Text(" HasMouseCursors");

imgui_internal.h

@@ -1321,8 +1321,8 @@ struct IMGUI_API ImGuiWindow
     ImVec2                  SetWindowPosVal;                    // store window position when using a non-zero Pivot (position set needs to be processed when we know the window size)
     ImVec2                  SetWindowPosPivot;                  // store window pivot for positioning. ImVec2(0,0) when positioning from top-left corner; ImVec2(0.5f,0.5f) for centering; ImVec2(1,1) for bottom right.
 
+    ImVector<ImGuiID>       IDStack;                            // ID stack. ID are hashes seeded with the value at the top of the stack. (In theory this should be in the TempData structure)
     ImGuiWindowTempData     DC;                                 // Temporary per-window data, reset at the beginning of the frame. This used to be called ImGuiDrawContext, hence the "DC" variable name.
-    ImVector<ImGuiID>       IDStack;                            // ID stack. ID are hashes seeded with the value at the top of the stack
 
     // The best way to understand what those rectangles are is to use the 'Metrics -> Tools -> Show windows rectangles' viewer.
     // The main 'OuterRect', omitted as a field, is window->Rect().
@@ -1334,6 +1334,7 @@ struct IMGUI_API ImGuiWindow
     ImRect                  ContentsRegionRect;                 // FIXME: This is currently confusing/misleading. It is essentially WorkRect but not handling of scrolling. We currently rely on it as right/bottom aligned sizing operation need some size to rely on.
 
     int                     LastFrameActive;                    // Last frame number the window was Active.
+    float                   LastTimeActive;
     float                   ItemWidthDefault;
     ImGuiMenuColumns        MenuColumns;                        // Simplified columns storage for menu items
     ImGuiStorage            StateStorage;
@@ -1352,6 +1353,10 @@ struct IMGUI_API ImGuiWindow
     ImGuiID                 NavLastIds[ImGuiNavLayer_COUNT];    // Last known NavId for this window, per layer (0/1)
     ImRect                  NavRectRel[ImGuiNavLayer_COUNT];    // Reference rectangle, in window relative space
 
+    bool                    MemoryCompacted;
+    int                     MemoryDrawListIdxCapacity;
+    int                     MemoryDrawListVtxCapacity;
+
 public:
     ImGuiWindow(ImGuiContext* context, const char* name);
     ~ImGuiWindow();
@@ -1483,6 +1488,8 @@ namespace ImGui
     IMGUI_API void          SetWindowPos(ImGuiWindow* window, const ImVec2& pos, ImGuiCond cond = 0);
     IMGUI_API void          SetWindowSize(ImGuiWindow* window, const ImVec2& size, ImGuiCond cond = 0);
     IMGUI_API void          SetWindowCollapsed(ImGuiWindow* window, bool collapsed, ImGuiCond cond = 0);
+    IMGUI_API void          GcCompactTransientWindowBuffers(ImGuiWindow* window);
+    IMGUI_API void          GcAwakeTransientWindowBuffers(ImGuiWindow* window);
 
     IMGUI_API void          SetCurrentFont(ImFont* font);
     inline ImFont*          GetDefaultFont() { ImGuiContext& g = *GImGui; return g.IO.FontDefault ? g.IO.FontDefault : g.IO.Fonts->Fonts[0]; }