libtxt: add boxes representing empty lines in GetRectsForRange (#5043)

If the start/end range passed to GetRectsForRange includes lines of text
that did not render any glyphs, then GetRectsForRange should add a placeholder
for that line.  In particular, the framework expects this so that it can
position the cursor on an empty line when editing text.

Fixes https://github.com/flutter/flutter/issues/16418

third_party/txt/src/txt/paragraph.cc

@@ -249,7 +249,7 @@ bool Paragraph::ComputeLineBreaks() {
     size_t block_size = block_end - block_start;
 
     if (block_size == 0) {
-      line_ranges_.emplace_back(block_start, block_start, true);
+      line_ranges_.emplace_back(block_start, block_end, block_end + 1, true);
       line_widths_.push_back(0);
       continue;
     }
@@ -297,8 +297,13 @@ bool Paragraph::ComputeLineBreaks() {
     const int* breaks = breaker_.getBreaks();
     for (size_t i = 0; i < breaks_count; ++i) {
       size_t break_start = (i > 0) ? breaks[i - 1] : 0;
-      line_ranges_.emplace_back(break_start + block_start,
-                                breaks[i] + block_start, i == breaks_count - 1);
+      size_t line_start = break_start + block_start;
+      size_t line_end = breaks[i] + block_start;
+      bool hard_break = i == breaks_count - 1;
+      size_t line_end_including_newline =
+          (hard_break && line_end < text_.size()) ? line_end + 1 : line_end;
+      line_ranges_.emplace_back(line_start, line_end,
+                                line_end_including_newline, hard_break);
       line_widths_.push_back(breaker_.getWidths()[i]);
     }
 
@@ -437,9 +442,9 @@ void Paragraph::Layout(double width, bool force) {
     std::vector<Range<size_t>> words;
     double word_gap_width = 0;
     size_t word_index = 0;
-    bool justify_line = (paragraph_style_.text_align == TextAlign::justify &&
-                         line_number != line_limit - 1 &&
-                         !line_ranges_[line_number].hard_break);
+    bool justify_line =
+        (paragraph_style_.text_align == TextAlign::justify &&
+         line_number != line_limit - 1 && !line_range.hard_break);
     FindWords(text_, line_range.start, line_range.end, &words);
     if (justify_line) {
       if (words.size() > 1) {
@@ -986,7 +991,7 @@ void Paragraph::PaintDecorations(SkCanvas* canvas, const PaintRecord& record) {
 
 std::vector<Paragraph::TextBox> Paragraph::GetRectsForRange(size_t start,
                                                             size_t end) const {
-  std::vector<TextBox> boxes;
+  std::map<size_t, std::vector<Paragraph::TextBox>> line_boxes;
 
   for (const CodeUnitRun& run : code_unit_runs_) {
     if (run.code_units.start >= end)
@@ -1014,10 +1019,34 @@ std::vector<Paragraph::TextBox> Paragraph::GetRectsForRange(size_t start,
       if (left == SK_ScalarMax || right == SK_ScalarMin)
         continue;
     }
-    boxes.emplace_back(SkRect::MakeLTRB(left, top, right, bottom),
-                       run.direction);
+    line_boxes[run.line_number].emplace_back(
+        SkRect::MakeLTRB(left, top, right, bottom), run.direction);
   }
 
+  // Add empty rectangles representing any line within the range that did not
+  // render any glyphs.
+  for (size_t line_number = 0; line_number < line_ranges_.size(); ++line_number) {
+    const LineRange& line = line_ranges_[line_number];
+    if (line.start >= end)
+      break;
+    if (line.end_including_newline <= start)
+      continue;
+    if (line_boxes.find(line_number) == line_boxes.end()) {
+      // If the range starts after the beginning of this line, then place the
+      // rectangle at the end of the line's width.  This is intended to
+      // handle ranges encompassing the newline character at the end of a line.
+      SkScalar x = (start > line.start) ? line_widths_[line_number] : 0;
+      SkScalar top = (line_number > 0) ? line_heights_[line_number - 1] : 0;
+      SkScalar bottom = line_heights_[line_number];
+      line_boxes[line_number].emplace_back(
+          SkRect::MakeLTRB(x, top, x, bottom), TextDirection::ltr);
+    }
+  }
+
+  std::vector<Paragraph::TextBox> boxes;
+  for (const auto& kv : line_boxes) {
+    boxes.insert(boxes.end(), kv.second.begin(), kv.second.end());
+  }
   return boxes;
 }
 

third_party/txt/src/txt/paragraph.h

@@ -63,8 +63,8 @@ class Paragraph {
   };
 
   struct TextBox {
-    const SkRect rect;
-    const TextDirection direction;
+    SkRect rect;
+    TextDirection direction;
 
     TextBox(SkRect r, TextDirection d) : rect(r), direction(d) {}
   };
@@ -187,8 +187,10 @@ class Paragraph {
   mutable std::unique_ptr<icu::BreakIterator> word_breaker_;
 
   struct LineRange {
-    LineRange(size_t s, size_t e, bool h) : start(s), end(e), hard_break(h) {}
+    LineRange(size_t s, size_t e, size_t ewn, bool h)
+        : start(s), end(e), end_including_newline(ewn), hard_break(h) {}
     size_t start, end;
+    size_t end_including_newline;
     bool hard_break;
   };
   std::vector<LineRange> line_ranges_;