Clipper: added UserIndex helper storage.

docs/CHANGELOG.txt

@@ -50,7 +50,10 @@ Breaking Changes:
 Other Changes:
 
 - Style: border sizes are now scaled (and rounded) by ScaleAllSizes().
-- Clipper: clear DisplayStart/DisplayEnd fields when Step() returns false.
+- Clipper:
+  - Clear `DisplayStart`/`DisplayEnd` fields when `Step()` returns false.
+  - Added `UserIndex` helper storage. This is solely a convenience for cases where
+    you may want to carry an index around.
 - Examples:
   - Emscripten: added `tabindex=-1` to canvas in our shell_minimal.htm. Without it,
     the canvas was not focusable in the DOM, which in turn make some backends

imgui.h

@@ -2853,6 +2853,7 @@ struct ImGuiListClipper
     ImGuiContext*   Ctx;                // Parent UI context
     int             DisplayStart;       // First item to display, updated by each call to Step()
     int             DisplayEnd;         // End of items to display (exclusive)
+    int             UserIndex;          // Helper storage for user convenience/code. Optional, and otherwise unused if you don't use it.
     int             ItemsCount;         // [Internal] Number of items
     float           ItemsHeight;        // [Internal] Height of item after a first step and item submission can calculate it
     double          StartPosY;          // [Internal] Cursor position at the time of Begin() or after table frozen rows are all processed