InputText: Fix using a combination of _CallbackResize + _EnterReturnsTrue + lack of persisting user storage. (#3009)

Amend 24ff25981 (#2006, #1443, #1008)

docs/CHANGELOG.txt

@@ -78,6 +78,10 @@ Other Changes:
 - ColorEdit: "Copy As" content-menu tool shows hex values both with/without alpha when available.
 - InputText: Fix corruption or crash when executing undo after clearing input with ESC, as a
   byproduct we are allowing to later undo the revert with a CTRL+Z. (#3008).
+- InputText: Fix using a combination of _CallbackResize (e.g. for std::string binding), along with the
+  _EnterReturnsTrue flag along with the rarely used property of using an InputText without persisting
+  user-side storage. Previously if you had e.g. a local unsaved std::string and reading result back
+  from the widget, the user string object wouldn't be resized when Enter key was pressed. (#3009)
 - MenuBar: Fix minor clipping issue where occasionally a menu text can overlap the right-most border.
 - Window: Fix SetNextWindowBgAlpha(1.0f) failing to override alpha component. (#3007) [@Albog]
 - Window: When testing for the presence of the ImGuiWindowFlags_NoBringToFrontOnFocus flag we

imgui_widgets.cpp

@@ -3857,7 +3857,8 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
         }
 
         // When using 'ImGuiInputTextFlags_EnterReturnsTrue' as a special case we reapply the live buffer back to the input buffer before clearing ActiveId, even though strictly speaking it wasn't modified on this frame.
-        // If we didn't do that, code like InputInt() with ImGuiInputTextFlags_EnterReturnsTrue would fail. Also this allows the user to use InputText() with ImGuiInputTextFlags_EnterReturnsTrue without maintaining any user-side storage.
+        // If we didn't do that, code like InputInt() with ImGuiInputTextFlags_EnterReturnsTrue would fail. 
+        // This also allows the user to use InputText() with ImGuiInputTextFlags_EnterReturnsTrue without maintaining any user-side storage (please note that if you use this property along ImGuiInputTextFlags_CallbackResize you can end up with your temporary string object unnecessarily allocating once a frame, either store your string data, either if you don't then don't use ImGuiInputTextFlags_CallbackResize).
         bool apply_edit_back_to_user_buffer = !cancel_edit || (enter_pressed && (flags & ImGuiInputTextFlags_EnterReturnsTrue) != 0);
         if (apply_edit_back_to_user_buffer)
         {
@@ -3951,8 +3952,11 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
         // Copy result to user buffer
         if (apply_new_text)
         {
+            // We cannot test for 'backup_current_text_length != apply_new_text_length' here because we have no guarantee that the size
+            // of our owned buffer matches the size of the string object held by the user, and by design we allow InputText() to be used
+            // without any storage on user's side.
             IM_ASSERT(apply_new_text_length >= 0);
-            if (backup_current_text_length != apply_new_text_length && is_resizable)
+            if (is_resizable)
             {
                 ImGuiInputTextCallbackData callback_data;
                 callback_data.EventFlag = ImGuiInputTextFlags_CallbackResize;
@@ -3967,6 +3971,7 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                 apply_new_text_length = ImMin(callback_data.BufTextLen, buf_size - 1);
                 IM_ASSERT(apply_new_text_length <= buf_size);
             }
+            //IMGUI_DEBUG_LOG("InputText(\"%s\"): apply_new_text length %d\n", label, apply_new_text_length);
 
             // If the underlying buffer resize was denied or not carried to the next frame, apply_new_text_length+1 may be >= buf_size.
             ImStrncpy(buf, apply_new_text, ImMin(apply_new_text_length + 1, buf_size));