[terminal] Fixes writing to a non-empty line sometimes destroying the contents of that line (#702).

Author: christianparpart

Changelog.md

@@ -1,6 +1,8 @@
 ### 0.3.2 (unreleased)
 
 - Adds E3 capability, so `clear` now defaults to clearing screen and scrollback (#693).
+- Fixes writing to a non-empty line sometimes destroying the contents of that line (#702).
+- Fixes underline decoration for wide character cells.
 - Fixes SGR 8 (Conceal/Hidden) attribute doesn't work as expected (#699).
 - Fixes Synchronized Updates (`SM/RM ? 2026`) sometimes lagging behind in rendering.
 - Fixes SGR and text breakage when altering charsets via `ESC ( 0` VT sequence (#661).
@@ -22,7 +24,6 @@
   - and more...
 - Adds specialized PTY implementation for Linux operating system utilizing OS-specific kernel APIs.
 - Changes CLI syntax for `contour parser-table` to `contour generate parser-table`.
-- Fixes underline decoration for wide character cells.
 
 ### 0.3.1 (2022-05-01)
 

src/terminal/Line.h

@@ -296,7 +296,7 @@ class Line
     }
     [[nodiscard]] bool isInflatedBuffer() const noexcept
     {
-        return std::holds_alternative<TrivialBuffer>(storage_);
+        return !std::holds_alternative<TrivialBuffer>(storage_);
     }
 
     void setBuffer(TrivialBuffer const& buffer) noexcept { storage_ = buffer; }

src/terminal/Screen.cpp

@@ -339,13 +339,35 @@ size_t Screen<Cell, TheScreenType>::emplaceCharsIntoCurrentLine(string_view _cha
     auto columnsAvailable = (_state.margin.horizontal.to.value + 1) - _state.cursor.position.column.value;
     auto const charsToWrite = static_cast<size_t>(min(columnsAvailable, static_cast<int>(_chars.size())));
 
-    currentLine().reset(_state.cursor.graphicsRendition,
-                        _state.cursor.hyperlink,
-                        crispy::BufferFragment {
-                            _terminal.currentPtyBuffer(),
-                            _chars.substr(0, charsToWrite),
-                        });
-    advanceCursorAfterWrite(ColumnCount::cast_from(charsToWrite));
+    Line<Cell>& line = currentLine();
+    if (!line.isInflatedBuffer() && line.empty())
+    {
+        // Only use fastpath if the currently line hasn't been inflated already.
+        // Because we might lose prior-written textual/SGR information otherwise.
+        line.reset(_state.cursor.graphicsRendition,
+                   _state.cursor.hyperlink,
+                   crispy::BufferFragment {
+                       _terminal.currentPtyBuffer(),
+                       _chars.substr(0, charsToWrite),
+                   });
+        advanceCursorAfterWrite(ColumnCount::cast_from(charsToWrite));
+    }
+    else
+    {
+        // Transforming _chars input from UTF-8 to UTF-32 even though right now it should only
+        // be containing US-ASCII, but soon it'll be any arbitrary textual Unicode codepoints.
+        auto utf8DecoderState = unicode::utf8_decoder_state {};
+        for (char const ch: _chars)
+        {
+            auto const result = unicode::from_utf8(utf8DecoderState, static_cast<uint8_t>(ch));
+            if (holds_alternative<unicode::Success>(result))
+                writeText(get<unicode::Success>(result).value);
+            else if (holds_alternative<unicode::Invalid>(result))
+                writeText(U'\uFFFE'); // U+FFFE (Not a Character)
+        }
+    }
+    // fmt::print("emplaceCharsIntoCurrentLine ({} cols, {} bytes): \"{}\"\n", cellCount, _chars.size(),
+    // _chars);
     return charsToWrite;
 }