Clipper: always pulls current context on ImGuiListClipper::Begin(). (#9324, #5856)

Marked Ctx as internal in the comments.

docs/CHANGELOG.txt

@@ -120,6 +120,8 @@ Other Changes:
   - 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.
+  - Always pulls current context on `ImGuiListClipper::Begin()`, consistent with public
+    API design, and avoids issues with clipper instances outliving contexts. (#9324, #5856)
 - Scrollbar:
   - Implemented a custom tweak to extend hit-testing bounding box when window is sitting
     at the edge of a viewport (e.g. fullscreen or docked window), so that e.g. mouse the

imgui.cpp

@@ -3270,8 +3270,7 @@ ImGuiListClipper::~ImGuiListClipper()
 
 void ImGuiListClipper::Begin(int items_count, float items_height)
 {
-    if (Ctx == NULL)
-        Ctx = ImGui::GetCurrentContext();
+    Ctx = ImGui::GetCurrentContext();
 
     ImGuiContext& g = *Ctx;
     ImGuiWindow* window = g.CurrentWindow;

imgui.h

@@ -2856,16 +2856,16 @@ enum ImGuiListClipperFlags_
 // - The clipper also handles various subtleties related to keyboard/gamepad navigation, wrapping etc.
 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
+    ImGuiListClipperFlags Flags;        // [Internal] Flags, currently not yet well exposed.
     double          StartPosY;          // [Internal] Cursor position at the time of Begin() or after table frozen rows are all processed
     double          StartSeekOffsetY;   // [Internal] Account for frozen rows in a table and initial loss of precision in very large windows.
+    ImGuiContext*   Ctx;                // [Internal] Parent UI context
     void*           TempData;           // [Internal] Internal data
-    ImGuiListClipperFlags Flags;        // [Internal] Flags, currently not yet well exposed.
 
     // items_count: Use INT_MAX if you don't know how many items you have (in which case the cursor won't be advanced in the final step, and you can call SeekCursorForItem() manually if you need)
     // items_height: Use -1.0f to be calculated automatically on first step. Otherwise pass in the distance between your items, typically GetTextLineHeightWithSpacing() or GetFrameHeightWithSpacing().