fix(input): advance the TextBox caret past trailing whitespace

The final cluster rect SDL_ttf reports stops at the last inked glyph, so a
caret at the end of a single-line field did not move when a trailing space
was typed. Use the laid-out width there; wrapped layouts keep the clamp
because their width is the widest line rather than the last one.

src/runtime/roblox_text_surface_overlay.cc

@@ -244,7 +244,7 @@ struct TextBytePosition {
 };
 
 TextBytePosition TextPosition(TTF_Text* layout, std::size_t byte_offset,
-                              int text_width) {
+                              int text_width, bool single_line) {
   if (layout == nullptr) {
     return {};
   }
@@ -259,7 +259,11 @@ TextBytePosition TextPosition(TTF_Text* layout, std::size_t byte_offset,
   position.y = substring.rect.y;
   position.height = substring.rect.h;
   if ((substring.flags & TTF_SUBSTRING_TEXT_END) != 0) {
-    position.x = std::min(text_width, substring.rect.x + substring.rect.w);
+    // Trailing whitespace has no ink, so the final cluster rect stops short of
+    // the laid-out width that a caret at the end of a single line sits behind.
+    position.x = single_line ? text_width
+                             : std::min(text_width,
+                                        substring.rect.x + substring.rect.w);
   }
   return position;
 }
@@ -501,11 +505,12 @@ Status RobloxTextSurfaceOverlay::RasterizeLocked() {
   TextBytePosition selection_end_position;
   if (layout != nullptr) {
     (void)TTF_GetTextSize(layout, &text_width, &text_height);
-    caret_position = TextPosition(layout, caret_byte, text_width);
-    selection_begin_position =
-        TextPosition(layout, selection_begin_byte, text_width);
+    caret_position =
+        TextPosition(layout, caret_byte, text_width, !wrapped_layout);
+    selection_begin_position = TextPosition(layout, selection_begin_byte,
+                                            text_width, !wrapped_layout);
     selection_end_position =
-        TextPosition(layout, selection_end_byte, text_width);
+        TextPosition(layout, selection_end_byte, text_width, !wrapped_layout);
   }
   if (!text.value.empty()) {
     // SDL_ttf implementations have historically differed on whether fg.a is

tests/roblox_text_display_state_test.cc

@@ -569,6 +569,31 @@ TEST(RobloxTextDisplayStateTest, WrapsMultilineTextInsideNativeTextBox) {
   EXPECT_TRUE(overlay.Shutdown().ok());
 }
 
+TEST(RobloxTextDisplayStateTest, AdvancesCaretForTrailingSpace) {
+  RobloxTextSurfaceOverlay overlay;
+  ASSERT_TRUE(overlay.Initialize({800, 600}).ok());
+  RobloxTextDisplaySink sink = overlay.sink();
+  const auto rightmost_pixel = [&](const std::string& text,
+                                   RobloxTextDisplayEvent event) {
+    RobloxTextDisplayUpdate update =
+        Show(1, text, static_cast<int32_t>(text.size()));
+    update.event = event;
+    update.font_size = 18.0F;
+    sink.update(sink.context, update);
+    MocktailTextOverlayFrameInfo frame;
+    EXPECT_TRUE(overlay.QueryFrame(&frame));
+    std::vector<std::uint8_t> rgba(frame.rgba_bytes);
+    EXPECT_TRUE(overlay.CopyFrame(frame.revision, rgba.data(), rgba.size()));
+    return FindAlphaBounds(frame, rgba).maximum_x;
+  };
+
+  const int typed = rightmost_pixel("hi", RobloxTextDisplayEvent::kShow);
+  const int spaced = rightmost_pixel("hi ", RobloxTextDisplayEvent::kUpdate);
+
+  EXPECT_GT(spaced, typed);
+  EXPECT_TRUE(overlay.Shutdown().ok());
+}
+
 }  // namespace
 }  // namespace runtime
 }  // namespace mocktail