Handle end-of-file newline semantics and improve scroll alignment logic.

.idea/workspace.xml

@@ -33,11 +33,9 @@
     </configurations>
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="e1fe3ab0-3650-4fca-8664-a247d5dfa457" name="Changes" comment="Add undo/redo infrastructure and buffer management additions.">
+    <list default="true" id="e1fe3ab0-3650-4fca-8664-a247d5dfa457" name="Changes" comment="Refactor `Buffer` to use `Line` abstraction and improve handling of row operations.&#10;&#10;This uses either a GapBuffer or PieceTable depending on the compilation.">
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/Buffer.cc" beforeDir="false" afterPath="$PROJECT_DIR$/Buffer.cc" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/Buffer.h" beforeDir="false" afterPath="$PROJECT_DIR$/Buffer.h" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/Command.cc" beforeDir="false" afterPath="$PROJECT_DIR$/Command.cc" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
     <option name="HIGHLIGHT_CONFLICTS" value="true" />
@@ -144,7 +142,7 @@
       <option name="number" value="Default" />
       <option name="presentableId" value="Default" />
       <updated>1764457173148</updated>
-      <workItem from="1764457174208" duration="27302000" />
+      <workItem from="1764457174208" duration="27972000" />
     </task>
     <task id="LOCAL-00001" summary="Add undo/redo infrastructure and buffer management additions.">
       <option name="closed" value="true" />
@@ -154,7 +152,15 @@
       <option name="project" value="LOCAL" />
       <updated>1764485311566</updated>
     </task>
-    <option name="localTasksCounter" value="2" />
+    <task id="LOCAL-00002" summary="Refactor `Buffer` to use `Line` abstraction and improve handling of row operations.&#10;&#10;This uses either a GapBuffer or PieceTable depending on the compilation.">
+      <option name="closed" value="true" />
+      <created>1764486011231</created>
+      <option name="number" value="00002" />
+      <option name="presentableId" value="LOCAL-00002" />
+      <option name="project" value="LOCAL" />
+      <updated>1764486011231</updated>
+    </task>
+    <option name="localTasksCounter" value="3" />
     <servers />
   </component>
   <component name="TypeScriptGeneratedFilesManager">
@@ -168,7 +174,8 @@
   <component name="VcsManagerConfiguration">
     <MESSAGE value="Refactoring" />
     <MESSAGE value="Add undo/redo infrastructure and buffer management additions." />
-    <option name="LAST_COMMIT_MESSAGE" value="Add undo/redo infrastructure and buffer management additions." />
+    <MESSAGE value="Refactor `Buffer` to use `Line` abstraction and improve handling of row operations.&#10;&#10;This uses either a GapBuffer or PieceTable depending on the compilation." />
+    <option name="LAST_COMMIT_MESSAGE" value="Refactor `Buffer` to use `Line` abstraction and improve handling of row operations.&#10;&#10;This uses either a GapBuffer or PieceTable depending on the compilation." />
   </component>
   <component name="XSLT-Support.FileAssociations.UIState">
     <expand />

Buffer.cc

@@ -42,13 +42,29 @@ Buffer::OpenFromFile(const std::string &path, std::string &err)
 		return false;
 	}
     
+    // Detect if file ends with a newline so we can preserve a final empty line
+    // in our in-memory representation (mg-style semantics).
+    bool ends_with_nl = false;
+    {
+        in.seekg(0, std::ios::end);
+        std::streamoff sz = in.tellg();
+        if (sz > 0) {
+            in.seekg(-1, std::ios::end);
+            char last = 0;
+            in.read(&last, 1);
+            ends_with_nl = (last == '\n');
+        } else {
+            in.clear();
+        }
+        // Rewind to start for line-by-line read
+        in.clear();
+        in.seekg(0, std::ios::beg);
+    }
+
     rows_.clear();
     std::string line;
     while (std::getline(in, line)) {
-     // std::getline strips the '\n', keep raw line
+        // std::getline strips the '\n', keep raw line content only
-     if (!line.empty() && !in.good()) {
-         // fallthrough
-     }
         // Handle potential Windows CRLF: strip trailing '\r'
         if (!line.empty() && line.back() == '\r') {
             line.pop_back();
@@ -56,8 +72,15 @@ Buffer::OpenFromFile(const std::string &path, std::string &err)
         rows_.emplace_back(line);
     }
 
-	// If file ends with a trailing newline, getline will have produced an empty
+    // If the file ended with a newline and we didn't already get an
-	// last line already. If the file is empty and no lines were read, keep rows_ empty.
+    // empty final row from getline (e.g., when the last textual line
+    // had content followed by '\n'), append an empty row to represent
+    // the cursor position past the last newline.
+    if (ends_with_nl) {
+        if (rows_.empty() || !rows_.back().empty()) {
+            rows_.emplace_back(std::string());
+        }
+    }
 
     nrows_          = rows_.size();
 	filename_       = path;

GUIRenderer.cc

@@ -83,7 +83,8 @@ GUIRenderer::Draw(Editor &ed)
 			long last_row = first_row + vis_rows - 1;
 
    // A) If user scrolled (scroll_y changed), and cursor outside, move cursor to nearest visible row
-			if (prev_scroll_y >= 0.0f && scroll_y != prev_scroll_y) {
+   // Skip this when we just forced a scroll alignment this frame (programmatic change).
+   if (!forced_scroll && prev_scroll_y >= 0.0f && scroll_y != prev_scroll_y) {
        long cyr = static_cast<long>(cy);
        if (cyr < first_row || cyr > last_row) {
            long new_row = (cyr < first_row) ? first_row : last_row;