Clipper: bunch of rework. (#3841, #1725)

- Focused/NavId now always included in display range.
- Any number of steps (while preserving zero-alloc policy).
- Non contiguous ranges for nav processing
- Moved new fields internally (+ moved StepNo away from sight so it doesn't get missused).
- Generally tweaks/refactors.

imgui.h

@@ -2172,52 +2172,45 @@ struct ImGuiStorage
 };
 
 // Helper: Manually clip large list of items.
-// If you are submitting lots of evenly spaced items and you have a random access to the list, you can perform coarse
-// clipping based on visibility to save yourself from processing those items at all.
+// If you have lots evenly spaced items and you have a random access to the list, you can perform coarse
+// clipping based on visibility to only submit items that are in view.
 // The clipper calculates the range of visible items and advance the cursor to compensate for the non-visible items we have skipped.
-// (Dear ImGui already clip items based on their bounds but it needs to measure text size to do so, whereas manual coarse clipping before submission makes this cost and your own data fetching/submission cost almost null)
+// (Dear ImGui already clip items based on their bounds but: it needs to first layout the item to do so, and generally
+//  fetching/submitting your own data incurs additional cost. Coarse clipping using ImGuiListClipper allows you to easily
+//  scale using lists with tens of thousands of items without a problem)
 // Usage:
 //   ImGuiListClipper clipper;
 //   clipper.Begin(1000);         // We have 1000 elements, evenly spaced.
-//   clipper.ForceDisplay(42);    // Optional, force element with given index to be displayed (use f.e. if you need to update a tooltip for a drag&drop source)
 //   while (clipper.Step())
 //       for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
 //           ImGui::Text("line number %d", i);
 // Generally what happens is:
 // - Clipper lets you process the first element (DisplayStart = 0, DisplayEnd = 1) regardless of it being visible or not.
-// - User code submit one element.
+// - User code submit that one element.
 // - Clipper can measure the height of the first element
 // - Clipper calculate the actual range of elements to display based on the current clipping rectangle, position the cursor before the first visible element.
 // - User code submit visible elements.
+// - The clipper also handles various subtleties related to keyboard/gamepad navigation, wrapping etc.
 struct ImGuiListClipper
 {
-    int     DisplayStart;
-    int     DisplayEnd;
-
-    // [Internal]
-    int     ItemsCount;
-    int     RangeStart[4];  // 1 for the user, rest for internal use
-    int     RangeEnd[4];
-    int     RangeCount;
-    int     YRangeMin[1];
-    int     YRangeMax[1];
-    int     YRangeCount;
-    int     StepNo;
-    int     ItemsFrozen;
-    float   ItemsHeight;
-    float   StartPosY;
-
-    IMGUI_API ImGuiListClipper();
-    IMGUI_API ~ImGuiListClipper();
+    int             DisplayStart;       // First item to display, updated by each call to Step()
+    int             DisplayEnd;         // End of items to display (exclusive)
+    int             ItemsCount;         // [Internal] Number of items
+    float           ItemsHeight;        // [Internal] Height of item after a first step and item submission can calculate it
+    float           StartPosY;          // [Internal] Cursor position at the time of Begin() or after table frozen rows are all processed
+    void*           TempData;           // [Internal] Internal data
 
     // 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)
     // items_height: Use -1.0f to be calculated automatically on first step. Otherwise pass in the distance between your items, typically GetTextLineHeightWithSpacing() or GetFrameHeightWithSpacing().
-    IMGUI_API void Begin(int items_count, float items_height = -1.0f);  // Automatically called by constructor if you passed 'items_count' or by Step() in Step 1.
-    IMGUI_API void End();                                               // Automatically called on the last call of Step() that returns false.
-    IMGUI_API void ForceDisplayRange(int item_start, int item_end);     // Optionally call before the first call to Step() if you need a range of items to be displayed regardless of visibility.
-    inline    void ForceDisplay(int item_start, int item_count = 1) { ForceDisplayRange(item_start, item_start + item_count); }  // Like ForceDisplayRange, but with a number instead of an end index.
-    IMGUI_API void ForceDisplayYRange(float y_min, float y_max);        // Like ForceDisplayRange, but with y coordinates instead of item indices.
-    IMGUI_API bool Step();                                              // Call until it returns false. The DisplayStart/DisplayEnd fields will be set and you can process/draw those items.
+    IMGUI_API ImGuiListClipper();
+    IMGUI_API ~ImGuiListClipper();
+    IMGUI_API void  Begin(int items_count, float items_height = -1.0f);
+    IMGUI_API void  End();             // Automatically called on the last call of Step() that returns false.
+    IMGUI_API bool  Step();            // Call until it returns false. The DisplayStart/DisplayEnd fields will be set and you can process/draw those items.
+
+    // Call ForceDisplayRangeXXX functions before first call to Step() if you need a range of items to be displayed regardless of visibility.
+    IMGUI_API void  ForceDisplayRangeByIndices(int item_min, int item_max); // item_max is exclusive e.g. use (42, 42+1) to make item 42 always visible BUT due to alignment/padding of certain items it is likely that an extra item may be included on either end of the display range.
+    IMGUI_API void  ForceDisplayRangeByPositions(float y_min, float y_max); // Absolute coordinates
 
 #ifndef IMGUI_DISABLE_OBSOLETE_FUNCTIONS
     inline ImGuiListClipper(int items_count, float items_height = -1.0f) { memset(this, 0, sizeof(*this)); ItemsCount = -1; Begin(items_count, items_height); } // [removed in 1.79]