InputText: Amends and tidying up: Fixed undo/redo state corruption wh…

…en editing buffer in user callback. (#4947, #4949)

docs/CHANGELOG.txt

@@ -88,7 +88,9 @@ Other Changes:
   trickled with the new input queue (happened on some backends only). (#2467, #1336)
 - InputText: Fixed a one-frame display glitch where pressing Escape to revert after a deletion
   would lead to small garbage being displayed for one frame. Curiously a rather old bug! (#3008)
-- InputText: Fixed an undo-state corruption issue when editing buffer before reactivating item. (#4947)
+- InputText: Fixed an undo-state corruption issue when editing main buffer before reactivating item. (#4947)
+- InputText: Fixed an undo-state corruption issue when editing in-flight buffer in user callback.
+  (#4947, #4949] [@JoshuaWebb]
 - Tables: Fixed incorrect border height used for logic when resizing one of several synchronized
   instance of a same table ID, when instances have a different height. (#3955).
 - Tables: Fixed incorrect auto-fit of parent windows when using non-resizable weighted columns. (#5276)

imgui.h

@@ -1809,6 +1809,7 @@ struct ImVector
     inline void         resize(int new_size, const T& v)    { if (new_size > Capacity) reserve(_grow_capacity(new_size)); if (new_size > Size) for (int n = Size; n < new_size; n++) memcpy(&Data[n], &v, sizeof(v)); Size = new_size; }
     inline void         shrink(int new_size)                { IM_ASSERT(new_size <= Size); Size = new_size; } // Resize a vector to a smaller size, guaranteed not to cause a reallocation
     inline void         reserve(int new_capacity)           { if (new_capacity <= Capacity) return; T* new_data = (T*)IM_ALLOC((size_t)new_capacity * sizeof(T)); if (Data) { memcpy(new_data, Data, (size_t)Size * sizeof(T)); IM_FREE(Data); } Data = new_data; Capacity = new_capacity; }
+    inline void         reserve_discard(int new_capacity)   { if (new_capacity <= Capacity) return; if (Data) IM_FREE(Data); Data = (T*)IM_ALLOC((size_t)new_capacity * sizeof(T)); Capacity = new_capacity; }
 
     // NB: It is illegal to call push_back/push_front/insert with a reference pointing inside the ImVector data itself! e.g. v.push_back(v[10]) is forbidden.
     inline void         push_back(const T& v)               { if (Size == Capacity) reserve(_grow_capacity(Size + 1)); memcpy(&Data[Size], &v, sizeof(v)); Size++; }

imgui_widgets.cpp

@@ -3929,46 +3929,39 @@ static bool InputTextFilterCharacter(unsigned int* p_char, ImGuiInputTextFlags f
     return true;
 }
 
-static inline void InputTextUpdateUndoStateAfterUserCallback(ImGuiInputTextState *state, const char *new_buf) {
-    // Find the shortest single replacement we can make to get the new text
-    // from the old text.
-    ImWchar *old_buf = state->TextW.Data;
-    int old_length = state->CurLenW;
-    int new_length = ImTextCountCharsFromUtf8(new_buf, NULL);
-    int shorter_length = ImMin(old_length, new_length);
-
-    int where = 0;
-    while (where < shorter_length) {
-        unsigned int b;
-        ImTextCharFromUtf8(&b, new_buf + where, NULL);
-        if (old_buf[where] != b)
+// Find the shortest single replacement we can make to get the new text from the old text.
+// Important: needs to be run before TextW is rewritten with the new characters because calling STB_TEXTEDIT_GETCHAR() at the end.
+// FIXME: Ideally we should transition toward (1) making InsertChars()/DeleteChars() update undo-stack (2) discourage (and keep reconcile) or obsolete (and remove reconcile) accessing buffer directly.
+static void InputTextReconcileUndoStateAfterUserCallback(ImGuiInputTextState* state, const char* new_buf_a, int new_length_a)
+{
+    ImGuiContext& g = *GImGui;
+    const ImWchar* old_buf = state->TextW.Data;
+    const int old_length = state->CurLenW;
+    const int new_length = ImTextCountCharsFromUtf8(new_buf_a, new_buf_a + new_length_a);
+    g.TempBuffer.reserve_discard((new_length + 1) * sizeof(ImWchar));
+    ImWchar* new_buf = (ImWchar*)(void*)g.TempBuffer.Data;
+    ImTextStrFromUtf8(new_buf, new_length + 1, new_buf_a, new_buf_a + new_length_a);
+
+    const int shorter_length = ImMin(old_length, new_length);
+    int first_diff;
+    for (first_diff = 0; first_diff < shorter_length; first_diff++)
+        if (old_buf[first_diff] != new_buf[first_diff])
             break;
-
-        where += 1;
-    }
+    if (first_diff == old_length && first_diff == new_length)
+        return;
 
     int old_last_diff = old_length - 1;
     int new_last_diff = new_length - 1;
-    while (old_last_diff >= where && new_last_diff >= 0) {
-        unsigned int b;
-        ImTextCharFromUtf8(&b, new_buf + new_last_diff, NULL);
-        if (old_buf[old_last_diff] != b)
+    for (; old_last_diff >= first_diff && new_last_diff >= first_diff; old_last_diff--, new_last_diff--)
+        if (old_buf[old_last_diff] != new_buf[new_last_diff])
             break;
 
-        old_last_diff -= 1;
-        new_last_diff -= 1;
-    }
-
-    int insert_len = new_last_diff - where + 1;
-    int delete_len = old_last_diff - where + 1;
-
-    if (insert_len > 0 || delete_len > 0) {
-        STB_TEXTEDIT_CHARTYPE *p = stb_text_createundo(&state->Stb.undostate, where, delete_len, insert_len);
-        if (p) {
-            for (int i=0; i < delete_len; ++i)
-                p[i] = ImStb::STB_TEXTEDIT_GETCHAR(state, where + i);
-        }
-    }
+    const int insert_len = new_last_diff - first_diff + 1;
+    const int delete_len = old_last_diff - first_diff + 1;
+    if (insert_len > 0 || delete_len > 0)
+        if (STB_TEXTEDIT_CHARTYPE* p = stb_text_createundo(&state->Stb.undostate, first_diff, delete_len, insert_len))
+            for (int i = 0; i < delete_len; i++)
+                p[i] = ImStb::STB_TEXTEDIT_GETCHAR(state, first_diff + i);
 }
 
 // Edit a string of text
@@ -4558,9 +4551,9 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                     if (buf_dirty)
                     {
                         IM_ASSERT(callback_data.BufTextLen == (int)strlen(callback_data.Buf)); // You need to maintain BufTextLen if you change the text!
-                        InputTextUpdateUndoStateAfterUserCallback(state, callback_data.Buf);
+                        InputTextReconcileUndoStateAfterUserCallback(state, callback_data.Buf, callback_data.BufTextLen); // FIXME: Move the rest of this block inside function and rename to InputTextReconcileStateAfterUserCallback() ?
                         if (callback_data.BufTextLen > backup_current_text_length && is_resizable)
-                            state->TextW.resize(state->TextW.Size + (callback_data.BufTextLen - backup_current_text_length));
+                            state->TextW.resize(state->TextW.Size + (callback_data.BufTextLen - backup_current_text_length)); // Worse case scenario resize
                         state->CurLenW = ImTextStrFromUtf8(state->TextW.Data, state->TextW.Size, callback_data.Buf, NULL);
                         state->CurLenA = callback_data.BufTextLen;  // Assume correct length and valid UTF-8 from user, saves us an extra strlen()
                         state->CursorAnimReset();