scan: show each phase as N of M, with progress, elapsed time and ETA, also while it waits (closes #112)

Every scan display names its phase and its place among the five, as
"[2/5] walk", with its progress, elapsed time and ETA. load counts the
records first; the walk estimates its total from the records under the
operands; the content phase adds up the sizes of the matching groups
and says it is searching until the first record returns. Each display
redraws every 100 ms on a terminal and every 5 seconds in plain mode.

On a terminal no line reaches the last column: a line too wide drops
its rate, then its bar. After the terminal is made narrower than the
widest line drawn, the display makes a new library object, whose erase
fits.

Judgement call: load and the content checks draw the bar too, once their totals are known.
Judgement call: the bar is 10 columns wide, not the width the line leaves.

Model: opus-5-5

README.md

@@ -770,7 +770,9 @@ Additional requirements:
   gives way to the spinner. Only when those four alone are too wide, as on a
   terminal narrower than about 70 columns hours into a scan of millions of
   files, is the line cut short at the terminal's edge, so the end of the ETA is
-  lost first.
+  lost first. A terminal made narrower while a phase runs gets the same: the
+  next draw erases the line within the new width and fits the line to it, and
+  the display goes on redrawing in place.
 - When stderr is not a terminal (a pipe, a file, `/dev/null`), do not emit ANSI
   redraws: print the line the moment each phase starts, every 5 seconds while it
   runs (whether or not an item has completed, and never more often), and once

TODO.md

@@ -37,12 +37,12 @@
   until the first one is returned. On a terminal a phase with an exact total
   draws its line followed by the library's bar, and any other a spinner followed
   by the line; every display is drawn again several times a second. No line
-  reaches the terminal's last column, however narrow: a line too wide leaves out
-  its rate, then its bar, keeping the phase, progress, elapsed time and ETA
-  whole, and is cut short only when those alone do not fit, as below about 70
-  columns late in a large scan. In plain mode the line is printed every 5
-  seconds whether or not an item completed (2026-10-16,
-  https://git.eeqj.de/sneak/sfdupes/issues/112)
+  reaches the terminal's last column, however narrow, also after the terminal is
+  made narrower during a phase: a line too wide leaves out its rate, then its
+  bar, keeping the phase, progress, elapsed time and ETA whole, and is cut short
+  only when those alone do not fit, as below about 70 columns late in a large
+  scan. In plain mode the line is printed every 5 seconds whether or not an item
+  completed (2026-10-16, https://git.eeqj.de/sneak/sfdupes/issues/112)
 
 - `scan`, `report` and `trees` first print a line on stderr naming the database
   they use, as an absolute path, and whether that path is the default or comes

progress.go

@@ -5,6 +5,7 @@ import (
 	"math"
 	"os"
 	"slices"
+	"strings"
 	"sync"
 	"time"
 
@@ -98,6 +99,7 @@ type progress struct {
 	mu         sync.Mutex
 	bar        *progressbar.ProgressBar // what the library draws; nil until then
 	withBar    bool                     // bar is the library's bar, not its spinner
+	widest     int                      // the widest line bar has drawn, in columns
 	label      string
 	total      int64
 	estimate   bool
@@ -176,15 +178,20 @@ func (p *progress) draw() {
 		text, withBar = p.line(width-barMargin, false)
 	}
 
+	margin := barMargin
 	if !withBar {
-		text, _ = p.line(width-spinnerMargin, true)
+		margin = spinnerMargin
+		text, _ = p.line(width-margin, true)
 	}
 
-	// The library makes a bar or a spinner. A display that turns from
-	// one to the other, as its line grows or the terminal is resized,
-	// gets a new one.
-	if p.bar == nil || p.withBar != withBar {
-		p.newBar(withBar)
+	// The library makes a bar or a spinner, and before each draw erases
+	// its line with as many spaces as the widest line it has drawn. A
+	// display gets a new one when it turns from one to the other, as its
+	// line grows or the terminal is resized, and when the terminal has
+	// been made too narrow for that erase, which would wrap and leave a
+	// line behind at every draw.
+	if p.bar == nil || p.withBar != withBar || p.widest >= width {
+		p.newBar(withBar, width)
 	}
 
 	if withBar {
@@ -192,16 +199,23 @@ func (p *progress) draw() {
 	}
 
 	p.bar.Describe(text)
+
+	// The library drew the text and its margin, less the terminal's last
+	// column, which the margin keeps clear.
+	p.widest = max(p.widest, len(text)+margin-1)
 }
 
 // newBar has the library make what draw draws: a bar for the phase's
 // total when withBar is set, and a spinner when not. Either shows the
 // text draw gives it, and nothing of its own besides the spinner, or
 // the bar and its percent. p.mu must be held.
-func (p *progress) newBar(withBar bool) {
+func (p *progress) newBar(withBar bool, width int) {
 	if p.bar != nil {
-		// Erase what was drawn before.
-		_ = p.bar.Clear()
+		// Erase what was drawn before, short of the last column of a
+		// terminal that may have been made narrower since: the library's
+		// own erase is as wide as the widest line it drew.
+		blank := strings.Repeat(" ", min(p.widest, width-1))
+		fmt.Fprint(os.Stderr, "\r"+blank+"\r")
 	}
 
 	total := int64(-1) // the library's spinner
@@ -220,7 +234,7 @@ func (p *progress) newBar(withBar bool) {
 		progressbar.OptionSetElapsedTime(false),
 		progressbar.OptionSetPredictTime(false),
 	)
-	p.withBar = withBar
+	p.withBar, p.widest = withBar, 0
 }
 
 // setTotal gives a phase that started without a total its total, once

progress_test.go

@@ -410,6 +410,60 @@ func TestTerminalLinesFitWidth(t *testing.T) {
 	}
 }
 
+// TestTerminalNarrowedRedrawsInPlace draws a large scan's display, two
+// and a half hours into the phase, on a terminal 120 columns wide, then
+// makes the terminal narrower than that line and draws it again a few
+// times. Nothing written after that, the erase before each redraw
+// included, may reach the narrower terminal's last column: it would
+// wrap, and every redraw would leave a line behind. At these widths the
+// spinner stays a spinner and the bar stays a bar.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestTerminalNarrowedRedrawsInPlace(t *testing.T) {
+	stderr := captureStderr(t)
+
+	cases := []struct {
+		params progressParams
+		narrow int
+	}{
+		{progressParams{label: scanPhase("walk"), total: -1}, 60},
+		{progressParams{label: scanPhase("hash"), total: 9876543}, 90},
+	}
+
+	for _, tc := range cases {
+		width := 120
+
+		// No timed draws: the test draws.
+		p := startProgress(tc.params, func() int { return width }, time.Hour)
+
+		p.mu.Lock()
+		p.count = 1234567
+		p.start = p.start.Add(-150 * time.Minute)
+		p.rateStart = p.start
+		p.mu.Unlock()
+
+		p.draw()
+
+		width = tc.narrow
+		before := len(stderr())
+
+		p.draw()
+		p.draw()
+		p.finish()
+
+		lines := strings.FieldsFuncSeq(stderr()[before:], func(r rune) bool {
+			return r == '\r' || r == '\n'
+		})
+
+		for line := range lines {
+			if n := utf8.RuneCountInString(line); n >= width {
+				t.Errorf("%d columns written on a terminal made %d wide: %q",
+					n, width, line)
+			}
+		}
+	}
+}
+
 // TestProgressWarningsOnOwnLines drives the terminal displays, spinner
 // and bar, through a run of warnings with no items between them, as when
 // the walk meets many unreadable paths, for many redraws: every warning