libtxt: fixes to text style inheritance (#4466)

* newly pushed styles should inherit from the top of the paragraph's style
  stack, not the most recently added style in StyledRuns
* make the paragraph-level style a default that is not pushed onto the stack
  and can not be popped

third_party/txt/src/txt/paragraph_builder.cc

@@ -34,37 +34,29 @@ ParagraphBuilder::~ParagraphBuilder() = default;
 
 void ParagraphBuilder::SetParagraphStyle(const ParagraphStyle& style) {
   paragraph_style_ = style;
-  // Keep a default style to fall back to.
-  TextStyle text_style;
-  text_style.font_weight = paragraph_style_.font_weight;
-  text_style.font_style = paragraph_style_.font_style;
-  text_style.font_family = paragraph_style_.font_family;
-  text_style.font_size = paragraph_style_.font_size;
-  PushStyle(text_style);
+  paragraph_style_index_ = runs_.AddStyle(style.GetTextStyle());
+  runs_.StartRun(paragraph_style_index_, text_.size());
 }
 
 void ParagraphBuilder::PushStyle(const TextStyle& style) {
-  const size_t text_index = text_.size();
-  runs_.EndRunIfNeeded(text_index);
-  const size_t style_index = runs_.AddStyle(style);
-  runs_.StartRun(style_index, text_index);
+  size_t style_index = runs_.AddStyle(style);
   style_stack_.push_back(style_index);
+  runs_.StartRun(style_index, text_.size());
 }
 
 void ParagraphBuilder::Pop() {
   if (style_stack_.empty())
     return;
-  const size_t text_index = text_.size();
-  runs_.EndRunIfNeeded(text_index);
   style_stack_.pop_back();
-  if (style_stack_.empty())
-    return;
-  const size_t style_index = style_stack_.back();
-  runs_.StartRun(style_index, text_index);
+  runs_.StartRun(PeekStyleIndex(), text_.size());
+}
+
+size_t ParagraphBuilder::PeekStyleIndex() const {
+  return style_stack_.size() ? style_stack_.back() : paragraph_style_index_;
 }
 
 const TextStyle& ParagraphBuilder::PeekStyle() const {
-  return runs_.PeekStyle();
+  return runs_.GetStyle(PeekStyleIndex());
 }
 
 void ParagraphBuilder::AddText(const std::u16string& text) {

third_party/txt/src/txt/paragraph_builder.h

@@ -78,6 +78,9 @@ class ParagraphBuilder {
   std::shared_ptr<FontCollection> font_collection_;
   StyledRuns runs_;
   ParagraphStyle paragraph_style_;
+  size_t paragraph_style_index_;
+
+  size_t PeekStyleIndex() const;
 
   FXL_DISALLOW_COPY_AND_ASSIGN(ParagraphBuilder);
 };

third_party/txt/src/txt/styled_runs.cc

@@ -47,11 +47,12 @@ size_t StyledRuns::AddStyle(const TextStyle& style) {
   return style_index;
 }
 
-const TextStyle& StyledRuns::PeekStyle() const {
-  return styles_.back();
+const TextStyle& StyledRuns::GetStyle(size_t style_index) const {
+  return styles_[style_index];
 }
 
 void StyledRuns::StartRun(size_t style_index, size_t start) {
+  EndRunIfNeeded(start);
   runs_.push_back(IndexedRun{style_index, start, start});
 }
 

third_party/txt/src/txt/styled_runs.h

@@ -50,8 +50,7 @@ class StyledRuns {
 
   size_t AddStyle(const TextStyle& style);
 
-  // Returns the last TextStyle on the stack.
-  const TextStyle& PeekStyle() const;
+  const TextStyle& GetStyle(size_t style_index) const;
 
   void StartRun(size_t style_index, size_t start);