InputText: Fixed various display corruption related to swapping the underlying buffer while a input widget is active (both for writable and read-only paths). Often they would manifest when manipulating the scrollbar of a multi-line input text.

docs/CHANGELOG.txt

@@ -41,6 +41,9 @@ Other Changes:
   meant to be square (to align with e.g. color button) we always use FramePadding.y. (#2367)
 - InputText: Fixed an edge case crash that would happen if another widget sharing the same ID
   is being swapped with an InputText that has yet to be activated.
+- InputText: Fixed various display corruption related to swapping the underlying buffer while
+  a input widget is active (both for writable and read-only paths). Often they would manifest
+  when manipulating the scrollbar of a multi-line input text.
 - TabBar: Fixed a crash when using BeginTabBar() recursively (didn't affect docking). (#2371)
 - TabBar: Added extra mis-usage error recovery. Past the assert, common mis-usage don't lead to
   hard crashes any more, facilitating integration with scripting languages. (#1651)

imgui_internal.h

@@ -572,10 +572,11 @@ struct IMGUI_API ImGuiMenuColumns
 struct IMGUI_API ImGuiInputTextState
 {
     ImGuiID                 ID;                     // widget id owning the text state
-    int                     CurLenW, CurLenA;       // we need to maintain our buffer length in both UTF-8 and wchar format.
+    int                     CurLenW, CurLenA;       // we need to maintain our buffer length in both UTF-8 and wchar format. UTF-8 len is valid even if TextA is not.
     ImVector<ImWchar>       TextW;                  // edit buffer, we need to persist but can't guarantee the persistence of the user-provided buffer. so we copy into own buffer.
     ImVector<char>          TextA;                  // temporary UTF8 buffer for callbacks and other operations. this is not updated in every code-path! size=capacity.
     ImVector<char>          InitialTextA;           // backup of end-user buffer at the time of focus (in UTF-8, unaltered)
+    bool                    TextAIsValid;           // temporary UTF8 buffer is not initially valid before we make the widget active (until then we pull the data from user argument)
     int                     BufCapacityA;           // end-user buffer capacity
     float                   ScrollX;                // horizontal scrolling/offset
     ImStb::STB_TexteditState Stb;                   // state for stb_textedit.h

imgui_widgets.cpp

@@ -3224,7 +3224,8 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
     bool select_all = (g.ActiveId != id) && ((flags & ImGuiInputTextFlags_AutoSelectAll) != 0 || user_nav_input_start) && (!is_multiline);
 
     const bool init_make_active = (focus_requested || user_clicked || user_scroll_finish || user_nav_input_start);
-    if (init_make_active && g.ActiveId != id)
+    const bool init_state = (init_make_active || user_scroll_active);
+    if (init_state && g.ActiveId != id)
     {
         // Access state even if we don't own it yet.
         state = &g.InputTextState;
@@ -3237,15 +3238,16 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
         memcpy(state->InitialTextA.Data, buf, buf_len + 1);
 
         // Start edition
-        const int prev_len_w = state->CurLenW;
         const char* buf_end = NULL;
         state->TextW.resize(buf_size + 1);          // wchar count <= UTF-8 count. we use +1 to make sure that .Data is always pointing to at least an empty string.
+        state->TextA.resize(0);
+        state->TextAIsValid = false;                // TextA is not valid yet (we will display buf until then)
         state->CurLenW = ImTextStrFromUtf8(state->TextW.Data, buf_size, buf, NULL, &buf_end);
         state->CurLenA = (int)(buf_end - buf);      // We can't get the result from ImStrncpy() above because it is not UTF-8 aware. Here we'll cut off malformed UTF-8.
 
         // Preserve cursor position and undo/redo stack if we come back to same widget
-        // FIXME: We should probably compare the whole buffer to be on the safety side. Comparing buf (utf8) and edit_state.Text (wchar).
-        const bool recycle_state = (state->ID == id) && (prev_len_w == state->CurLenW);
+        // FIXME: For non-readonly widgets we might be able to require that TextAIsValid && TextA == buf ? (untested) and discard undo stack if user buffer has changed.
+        const bool recycle_state = (state->ID == id);
         if (recycle_state)
         {
             // Recycle existing cursor/selection/undo stack but clamp position
@@ -3266,7 +3268,7 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
             select_all = true;
     }
 
-    if (init_make_active)
+    if (g.ActiveId != id && init_make_active)
     {
         IM_ASSERT(state && state->ID == id);
         SetActiveID(id, window);
@@ -3282,7 +3284,7 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
         ClearActiveID();
 
     // Release focus when we click outside
-    if (!init_make_active && io.MouseClicked[0])
+    if (g.ActiveId == id && io.MouseClicked[0] && !init_state && !init_make_active)
         clear_active_id = true;
 
     bool value_changed = false;
@@ -3290,15 +3292,20 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
     int backup_current_text_length = 0;
 
     // When read-only we always use the live data passed to the function
-    if (g.ActiveId == id && is_readonly && !g.ActiveIdIsJustActivated)
+    // FIXME-OPT: Because our selection/cursor code currently needs the wide text we need to convert it when active, which is not ideal :(
+    if (is_readonly && state != NULL)
+    {
+        const bool will_render_cursor = (g.ActiveId == id) || (state && user_scroll_active);
+        const bool will_render_selection = state && state->HasSelection() && (RENDER_SELECTION_WHEN_INACTIVE || will_render_cursor);
+        if (will_render_cursor || will_render_selection)
         {
-        IM_ASSERT(state != NULL);
             const char* buf_end = NULL;
             state->TextW.resize(buf_size + 1);
             state->CurLenW = ImTextStrFromUtf8(state->TextW.Data, state->TextW.Size, buf, NULL, &buf_end);
             state->CurLenA = (int)(buf_end - buf);
             state->CursorClamp();
         }
+    }
 
     // Process mouse inputs and character inputs
     if (g.ActiveId == id)
@@ -3516,6 +3523,7 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
             // FIXME-OPT: CPU waste to do this every time the widget is active, should mark dirty state from the stb_textedit callbacks.
             if (!is_readonly)
             {
+                state->TextAIsValid = true;
                 state->TextA.resize(state->TextW.Size * 4 + 1);
                 ImTextStrToUtf8(state->TextA.Data, state->TextA.Size, state->TextW.Data, NULL);
             }
@@ -3646,11 +3654,8 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
     // without any carriage return, which would makes ImFont::RenderText() reserve too many vertices and probably crash. Avoid it altogether.
     // Note that we only use this limit on single-line InputText(), so a pathologically large line on a InputTextMultiline() would still crash.
     const int buf_display_max_length = 2 * 1024 * 1024;
-
-    // Select which buffer we are going to display. We set buf to NULL to prevent accidental usage from now on.
-    const char* buf_display = (g.ActiveId == id && state && !is_readonly) ? state->TextA.Data : buf;
-    IM_ASSERT(buf_display);
-    buf = NULL;
+    const char* buf_display = NULL;
+    const char* buf_display_end = NULL;
 
     // Render text. We currently only render selection when the widget is active or while scrolling.
     // FIXME: We could remove the '&& render_cursor' to keep rendering selection when inactive.
@@ -3790,9 +3795,10 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
         }
 
         // We test for 'buf_display_max_length' as a way to avoid some pathological cases (e.g. single-line 1 MB string) which would make ImDrawList crash.
-        const int buf_display_len = state->CurLenA;
-        if (is_multiline || buf_display_len < buf_display_max_length)
-            draw_window->DrawList->AddText(g.Font, g.FontSize, draw_pos - draw_scroll, GetColorU32(ImGuiCol_Text), buf_display, buf_display + buf_display_len, 0.0f, is_multiline ? NULL : &clip_rect);
+        buf_display = (!is_readonly && state->TextAIsValid) ? state->TextA.Data : buf;
+        buf_display_end = buf_display + state->CurLenA;
+        if (is_multiline || (buf_display_end - buf_display) < buf_display_max_length)
+            draw_window->DrawList->AddText(g.Font, g.FontSize, draw_pos - draw_scroll, GetColorU32(ImGuiCol_Text), buf_display, buf_display_end, 0.0f, is_multiline ? NULL : &clip_rect);
 
         // Draw blinking cursor
         if (render_cursor)
@@ -3812,9 +3818,11 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
     else
     {
         // Render text only (no selection, no cursor)
-        const char* buf_display_end = NULL;
+        buf_display = (g.ActiveId == id && !is_readonly && state->TextAIsValid) ? state->TextA.Data : buf;
         if (is_multiline)
             text_size = ImVec2(size.x, InputTextCalcTextLenAndLineCount(buf_display, &buf_display_end) * g.FontSize); // We don't need width
+        else if (g.ActiveId == id)
+            buf_display_end = buf_display + state->CurLenA;
         else
             buf_display_end = buf_display + strlen(buf_display);
         if (is_multiline || (buf_display_end - buf_display) < buf_display_max_length)
@@ -3833,7 +3841,7 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
 
     // Log as text
     if (g.LogEnabled && !is_password)
-        LogRenderedText(&draw_pos, buf_display, NULL);
+        LogRenderedText(&draw_pos, buf_display, buf_display_end);
 
     if (label_size.x > 0)
         RenderText(ImVec2(frame_bb.Max.x + style.ItemInnerSpacing.x, frame_bb.Min.y + style.FramePadding.y), label);