InputText: Fixed updating cursor/selection position when a callback alters the buffer in a way where the byte count is unchanged but the decoded character count changes. (#3587)

docs/CHANGELOG.txt

@@ -64,9 +64,11 @@ Other Changes:
 - Drag and Drop: Fix drag and drop to tie same-size drop targets by choosen the later one. Fixes dragging
   into a full-window-sized dockspace inside a zero-padded window. (#3519, #2717) [@Black-Cat]
 - Checkbox: Added CheckboxFlags() helper with int* type.
+- InputText: Fixed updating cursor/selection position when a callback altered the buffer in a way
+  where the byte count is unchanged but the decoded character count changes. (#3587) [@gqw]
 - Metrics: Fixed mishandling of ImDrawCmd::VtxOffset in wireframe mesh renderer.
 - Misc: Replaced UTF-8 decoder by branchless one by Christopher Wellons (30~40% faster). [@rokups]
-  Super minor fix handling incomplete UTF-8 contents: if input does not contain enough bytes, decoder 
+  Super minor fix handling incomplete UTF-8 contents: if input does not contain enough bytes, decoder
   returns IM_UNICODE_CODEPOINT_INVALID and consume remaining bytes (vs old decoded consumed only 1 byte).
 - Backends: OpenGL3: Use glGetString(GL_VERSION) query instead of glGetIntegerv(GL_MAJOR_VERSION, ...)
   when the later returns zero (e.g. Desktop GL 2.x). (#3530) [@xndcn]

imgui_widgets.cpp

@@ -4273,10 +4273,11 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                     IM_ASSERT(callback_data.Buf == state->TextA.Data);  // Invalid to modify those fields
                     IM_ASSERT(callback_data.BufSize == state->BufCapacityA);
                     IM_ASSERT(callback_data.Flags == flags);
-                    if (callback_data.CursorPos != utf8_cursor_pos)            { state->Stb.cursor = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.CursorPos); state->CursorFollow = true; }
-                    if (callback_data.SelectionStart != utf8_selection_start)  { state->Stb.select_start = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.SelectionStart); }
-                    if (callback_data.SelectionEnd != utf8_selection_end)      { state->Stb.select_end = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.SelectionEnd); }
-                    if (callback_data.BufDirty)
+                    const bool buf_dirty = callback_data.BufDirty;
+                    if (callback_data.CursorPos != utf8_cursor_pos || buf_dirty)            { state->Stb.cursor = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.CursorPos); state->CursorFollow = true; }
+                    if (callback_data.SelectionStart != utf8_selection_start || buf_dirty)  { state->Stb.select_start = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.SelectionStart); }
+                    if (callback_data.SelectionEnd != utf8_selection_end || buf_dirty)      { state->Stb.select_end = ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.SelectionEnd); }
+                    if (buf_dirty)
                     {
                         IM_ASSERT(callback_data.BufTextLen == (int)strlen(callback_data.Buf)); // You need to maintain BufTextLen if you change the text!
                         if (callback_data.BufTextLen > backup_current_text_length && is_resizable)