Rework termstatus

This now keeps the cursor at the first column of the first status line
so that messages printed to stdout or stderr by some other part of the
progarm will still be visible. The message will overwrite the status
lines, but those are easily reprinted on the next status update.

internal/ui/termstatus/terminal_posix.go

@@ -1,33 +1,36 @@
 package termstatus
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"os"
 )
 
 const (
-	posixMoveCursorHome = "\r"
-	posixMoveCursorUp   = "\x1b[1A"
-	posixClearLine      = "\x1b[2K"
+	posixControlMoveCursorHome = "\r"
+	posixControlMoveCursorUp   = "\x1b[1A"
+	posixControlClearLine      = "\x1b[2K"
 )
 
-// posixClearLines will clear the current line and the n lines above.
-// Afterwards the cursor is positioned at the start of the first cleared line.
-func posixClearLines(wr io.Writer, fd uintptr, n int) {
+// posixClearCurrentLine removes all characters from the current line and resets the
+// cursor position to the first column.
+func posixClearCurrentLine(wr io.Writer, fd uintptr) {
 	// clear current line
-	_, err := wr.Write([]byte(posixMoveCursorHome + posixClearLine))
+	_, err := wr.Write([]byte(posixControlMoveCursorHome + posixControlClearLine))
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "write failed: %v\n", err)
 		return
 	}
+}
 
-	for ; n > 0; n-- {
-		// clear current line and move on line up
-		_, err := wr.Write([]byte(posixMoveCursorUp + posixClearLine))
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "write failed: %v\n", err)
-			return
-		}
+// posixMoveCursorUp moves the cursor to the line n lines above the current one.
+func posixMoveCursorUp(wr io.Writer, fd uintptr, n int) {
+	data := []byte(posixControlMoveCursorHome)
+	data = append(data, bytes.Repeat([]byte(posixControlMoveCursorUp), n)...)
+	_, err := wr.Write(data)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "write failed: %v\n", err)
+		return
 	}
 }