unix: Fall back to reading LINES/COLUMNS in Terminal::get_dimensions

The ioctl to get the window size may fail silently by returning zeroed
rows/columns which are nonsensical and unusable by terminal
applications. In this case Vim falls back to attempting to read the
LINES and COLUMNS environment variables, and then attempts to read
dimensions from the terminfo. That fallback can be done in the calling
application however.

Also see <https://github.com/helix-editor/helix/issues/14101#issuecomment-3149628173>

src/terminal/unix.rs

@@ -159,7 +159,32 @@ impl Terminal for UnixTerminal {
 
     fn get_dimensions(&self) -> io::Result<WindowSize> {
         let winsize = termios::tcgetwinsize(self.write.get_ref())?;
-        Ok(winsize.into())
+        let mut size: WindowSize = winsize.into();
+        // Over a serial connection for example, the ioctl may quietly fail by returning zeroed
+        // rows and columns. Fall back to reading LINES/COLUMNS.
+        // <https://github.com/vim/vim/blob/b88f9e4a04ce9fb70abb7cdae17688aa4f49c8c9/src/os_unix.c#L4349-L4370>
+        if size.cols == 0 || size.rows == 0 {
+            if let Some(rows) = std::env::var("LINES")
+                .ok()
+                .and_then(|l| l.parse::<u16>().ok())
+            {
+                size.rows = rows;
+            }
+            if let Some(cols) = std::env::var("COLUMNS")
+                .ok()
+                .and_then(|c| c.parse::<u16>().ok())
+            {
+                size.cols = cols;
+            }
+        }
+        if size.cols == 0 || size.rows == 0 {
+            Err(io::Error::new(
+                io::ErrorKind::Other,
+                "cannot read non-zero cols/rows from ioctl or COLUMNS/LINES environment variables",
+            ))
+        } else {
+            Ok(size)
+        }
     }
 
     fn event_reader(&self) -> EventReader {