Fixed `GetBackgroundDrawList()`/`GetForegroundDrawList()` not being properly reset every frame when cumulated session time is very large. (#9521) Using a FrameCount for compare. Kept time as float to be consistent with other GC-related timestamps: if there is an actual need to use double we'll switch everywhere.

diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 14978c6..21f4524 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt

@@ -56,6 +56,9 @@
   - Reworked `AddFontDefault()` to use `io.DisplayFrameBufferScale` as part of the heuristic
     to select `AddFontDefaultVector()` by default, effectively using ProggyForever instead of
     ProggyClean on most Apple/Retina setups by default.
+- Misc:
+  - Fixed `GetBackgroundDrawList()`/`GetForegroundDrawList()` not being properly reset
+    every frame when cumulated session time is very large (e.g. a few days). [@lailoken]
 - ImDrawList:
   - Per-viewport FramebufferScale for Apple/Retina screen is applied to draw lists:
     - AA Fringe is scaled accordingly.

diff --git a/imgui.cpp b/imgui.cpp
index d157353..c4ac847 100644
--- a/imgui.cpp
+++ b/imgui.cpp

@@ -5305,12 +5305,13 @@
     }
 
     // Our ImDrawList system requires that there is always a command
-    if (viewport->BgFgDrawListsLastTimeActive[drawlist_no] != (float)g.Time)
+    if (viewport->BgFgDrawListsLastFrameActive[drawlist_no] != g.FrameCount)
     {
         draw_list->_ResetForNewFrame();
         draw_list->_SetPixelDensity(viewport->FramebufferScale.x);
         draw_list->PushTexture(g.IO.Fonts->TexRef);
         draw_list->PushClipRect(viewport->Pos, viewport->Pos + viewport->Size, false);
+        viewport->BgFgDrawListsLastFrameActive[drawlist_no] = g.FrameCount;
         viewport->BgFgDrawListsLastTimeActive[drawlist_no] = (float)g.Time;
     }
     return draw_list;
@@ -6212,7 +6213,7 @@
     for (ImGuiViewportP* viewport : g.Viewports)
     {
         InitViewportDrawData(viewport);
-        if (viewport->BgFgDrawLists[0] != NULL && viewport->BgFgDrawListsLastTimeActive[0] == (float)g.Time)
+        if (viewport->BgFgDrawLists[0] != NULL && viewport->BgFgDrawListsLastFrameActive[0] == g.FrameCount)
             AddDrawListToDrawDataEx(&viewport->DrawDataP, viewport->DrawDataBuilder.Layers[0], GetBackgroundDrawList(viewport));
     }
 
@@ -6244,7 +6245,7 @@
         FlattenDrawDataIntoSingleLayer(&viewport->DrawDataBuilder);
 
         // Add foreground ImDrawList (for each active viewport)
-        if (viewport->BgFgDrawLists[1] != NULL && viewport->BgFgDrawListsLastTimeActive[1] == (float)g.Time)
+        if (viewport->BgFgDrawLists[1] != NULL && viewport->BgFgDrawListsLastFrameActive[1] == g.FrameCount)
             AddDrawListToDrawDataEx(&viewport->DrawDataP, viewport->DrawDataBuilder.Layers[0], GetForegroundDrawList(viewport));
 
         // We call _PopUnusedDrawCmd() last thing, as RenderDimmedBackgrounds() rely on a valid command being there (especially in docking branch).

diff --git a/imgui_internal.h b/imgui_internal.h
index ef5bf9c..6312164 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h

@@ -1999,8 +1999,9 @@
 // Every instance of ImGuiViewport is in fact a ImGuiViewportP.
 struct ImGuiViewportP : public ImGuiViewport
 {
-    float               BgFgDrawListsLastTimeActive[2]; // Last frame number the background (0) and foreground (1) draw lists were used
     ImDrawList*         BgFgDrawLists[2];       // Convenience background (0) and foreground (1) draw lists. We use them to draw software mouser cursor when io.MouseDrawCursor is set and to draw most debug overlays.
+    int                 BgFgDrawListsLastFrameActive[2]; // Last frame the background (0) and foreground (1) draw lists were used.
+    float               BgFgDrawListsLastTimeActive[2];  // Timestamps for Gc.
     ImDrawData          DrawDataP;
     ImDrawDataBuilder   DrawDataBuilder;        // Temporary data while building final ImDrawData
 
@@ -2013,7 +2014,7 @@
     ImVec2              BuildWorkInsetMin;      // Work Area inset accumulator for current frame, to become next frame's WorkInset
     ImVec2              BuildWorkInsetMax;      // "
 
-    ImGuiViewportP()    { BgFgDrawListsLastTimeActive[0] = BgFgDrawListsLastTimeActive[1] = -1.0f; BgFgDrawLists[0] = BgFgDrawLists[1] = NULL; }
+    ImGuiViewportP()    { BgFgDrawLists[0] = BgFgDrawLists[1] = NULL; BgFgDrawListsLastFrameActive[0] = BgFgDrawListsLastFrameActive[1] = -1; BgFgDrawListsLastTimeActive[0] = BgFgDrawListsLastTimeActive[1] = -1.0; }
     ~ImGuiViewportP()   { if (BgFgDrawLists[0]) IM_DELETE(BgFgDrawLists[0]); if (BgFgDrawLists[1]) IM_DELETE(BgFgDrawLists[1]); }
 
     // Calculate work rect pos/size given a set of offset (we have 1 pair of offset for rect locked from last frame data, and 1 pair for currently building rect)