Fix O(n²) text cursor search with UTF-8 aware O(n)

src/core.c

@@ -464,6 +464,24 @@ float iui_text_width_vec(const char *text, float font_height)
     return w;
 }
 
+/* Codepoint width measurement for built-in vector font.
+ * Returns the width of a Unicode codepoint at the given font height.
+ * Codepoints outside ASCII range (0x20-0x7E) use the fallback box glyph.
+ */
+float iui_codepoint_width_vec(uint32_t cp, float font_height)
+{
+    if (font_height <= 0.f)
+        return 0.f;
+
+    float scale = font_height / IUI_FONT_UNITS_PER_EM;
+    const float pen_w = iui_vector_pen_for_height(font_height);
+    const float side = iui_vector_side_bearing(scale, pen_w);
+    /* iui_get_glyph handles out-of-range codepoints by returning box glyph */
+    const signed char *g = iui_get_glyph((unsigned char) (cp > 0x7F ? 0 : cp));
+    float glyph_w = (float) (IUI_GLYPH_RIGHT(g) - IUI_GLYPH_LEFT(g)) * scale;
+    return glyph_w + side * 2.f;
+}
+
 /* Emit vector commands for drawing a glyph.
  * @ctx:     Current UI context
  * @g:       Pointer to glyph data

src/draw.c

@@ -145,6 +145,42 @@ float iui_get_text_width(iui_context *ctx, const char *text)
     return width;
 }
 
+/* Get width of a Unicode codepoint. Uses renderer callback if available,
+ * otherwise falls back to built-in vector font.
+ *
+ * Note: For custom renderers, this measures single codepoints independently.
+ * Fonts with kerning may show slight cursor drift vs rendered text. For the
+ * built-in vector font (no kerning), width accumulation is exact.
+ */
+float iui_get_codepoint_width(iui_context *ctx, uint32_t cp)
+{
+    /* Clamp invalid codepoints to replacement character */
+    if (cp > 0x10FFFF)
+        cp = 0xFFFD;
+
+    if (ctx->renderer.text_width) {
+        /* Encode codepoint to UTF-8 for renderer callback */
+        char tmp[5] = {0};
+        if (cp < 0x80) {
+            tmp[0] = (char) cp;
+        } else if (cp < 0x800) {
+            tmp[0] = (char) (0xC0 | (cp >> 6));
+            tmp[1] = (char) (0x80 | (cp & 0x3F));
+        } else if (cp < 0x10000) {
+            tmp[0] = (char) (0xE0 | (cp >> 12));
+            tmp[1] = (char) (0x80 | ((cp >> 6) & 0x3F));
+            tmp[2] = (char) (0x80 | (cp & 0x3F));
+        } else {
+            tmp[0] = (char) (0xF0 | (cp >> 18));
+            tmp[1] = (char) (0x80 | ((cp >> 12) & 0x3F));
+            tmp[2] = (char) (0x80 | ((cp >> 6) & 0x3F));
+            tmp[3] = (char) (0x80 | (cp & 0x3F));
+        }
+        return ctx->renderer.text_width(tmp, ctx->renderer.user);
+    }
+    return iui_codepoint_width_vec(cp, ctx->font_height);
+}
+
 void iui_internal_draw_text(iui_context *ctx,
                             float x,
                             float y,

src/input.c

@@ -284,7 +284,9 @@ iui_textfield_result iui_textfield(iui_context *ctx,
  * @text_x_start: Starting x position of text
  * @click_x:      X coordinate of mouse click
  *
- * Returns position in buffer closest to click_x.
+ * Returns byte position in buffer closest to click_x.
+ * Uses incremental width computation for O(n) complexity.
+ * Iterates by UTF-8 codepoint to ensure cursor lands on valid boundaries.
  */
 static size_t iui_find_cursor_from_x(iui_context *ctx,
                                      const char *buffer,
@@ -295,20 +297,26 @@ static size_t iui_find_cursor_from_x(iui_context *ctx,
     if (len == 0)
         return 0;
 
-    /* Binary search would be ideal, but linear scan is fine for typical lengths
-     */
     float best_dist = fabsf(click_x - text_x_start);
+    float cumulative_x = text_x_start;
+    size_t pos = 0;
 
-    char tmp[IUI_STRING_BUFFER_SIZE];
-    for (size_t i = 1; i <= len && i < sizeof(tmp); i++) {
-        memcpy(tmp, buffer, i);
-        tmp[i] = '\0';
-        float char_x = text_x_start + iui_get_text_width(ctx, tmp);
-        float dist = fabsf(click_x - char_x);
+    while (pos < len) {
+        /* Decode codepoint at current position */
+        uint32_t cp = iui_utf8_decode(buffer, pos, len);
+        size_t next_pos = iui_utf8_next(buffer, pos, len);
+
+        /* Accumulate width for this codepoint */
+        cumulative_x += iui_get_codepoint_width(ctx, cp);
+
+        /* Check if this boundary is closer to click position */
+        float dist = fabsf(click_x - cumulative_x);
         if (dist < best_dist) {
             best_dist = dist;
-            best_pos = i;
+            best_pos = next_pos;
         }
+
+        pos = next_pos;
     }
     return best_pos;
 }

src/internal.h

@@ -969,6 +969,7 @@ static inline bool iui_process_text_input(iui_context *ctx,
 
 /* Text rendering helpers (implemented in iui_draw.c) */
 float iui_get_text_width(iui_context *ctx, const char *text);
+float iui_get_codepoint_width(iui_context *ctx, uint32_t cp);
 void iui_internal_draw_text(iui_context *ctx,
                             float x,
                             float y,
@@ -1009,6 +1010,7 @@ void iui_compute_vector_metrics(float font_height,
                                 float *out_ascent_px,
                                 float *out_descent_px);
 float iui_vector_pen_for_height(float font_height);
+float iui_codepoint_width_vec(uint32_t cp, float font_height);
 
 /* Theme globals (defined in iui_core.c) */
 extern const iui_theme_t g_theme_light;