Text selection doesn't work inside full-screen programs — alternate-screen scroll detaches buffer lines

[lordspace]

Symptom

Selecting text with the mouse does nothing while a full-screen program (less,
htop, an editor) is on screen: no highlight, no selection, nothing copied.
Double-click and right-click keep working, which makes it look like a gesture
problem — it isn't.

Reproduced on macOS and Linux, with a mouse and a trackpad, on xterm 4.0.0 (the
current release) and on master.

Reproduction

final terminal = Terminal(maxLines: 10000);
final controller = TerminalController();

terminal.resize(80, 24);
terminal.write('\x1b[?1049h');          // alternate screen

for (var row = 0; row < 40; row++) {    // paint past the last row => scrolls
  terminal.write('row $row\r\n');
}

expect(terminal.buffer.lines[0].attached, isTrue);   // FAILS: false

final base = terminal.buffer.createAnchorFromOffset(const CellOffset(0, 23));
final extent = terminal.buffer.createAnchorFromOffset(const CellOffset(16, 23));

controller.setSelection(base, extent);

expect(controller.selection, isNotNull);             // FAILS: null

Root cause

Buffer.scrollUp (lib/src/core/buffer/buffer.dart) shifts every line up a
slot through the index setter:

this.lines[i] = this.lines[i + lines];

That attaches the line at i but leaves a stale duplicate reference at
i + lines. _adoptChild (lib/src/utils/circular_buffer.dart) then detaches
whatever occupies the target slot unconditionally:

void _adoptChild(int index, T child) {
  final cyclicIndex = _getCyclicIndex(index);
  _array[cyclicIndex]?._detach();                 // <-- detaches the re-homed line
  _array[cyclicIndex] = child.._attach(this, index);
}

So the next iteration's adopt into i + 1 detaches the line that was just
re-homed at i. After a single alternate-screen scroll, effectively every
visible line is detached.

TerminalController.selection returns null when either anchor is detached, so
every drag sets a selection that immediately reads back as nothing. The gesture
layer is fine — the selection pan fires on every drag update and calls
setSelection each time.

Evidence from a running app

A probe on click, printing the clicked cell plus the attachment state:

alt=false  cell=(43,24)  lineAttached=true   anchorAttached=true    (a shell selects fine)
alt=true   cell=(4,2)    lineAttached=false  anchorAttached=false   (nothing selects)

Cell mapping is correct in both cases; only attachment differs.

Why double-click still appears to work

Only the LAST row stays reachable in practice, which is why a double-click on a
pager's status line looks like it works while dragging anywhere does nothing.

Fix

Detach the previous occupant only when this slot is still its home. _attach
already rewrites _absoluteIndex, so a re-homed item maps to a different slot
and is left alone, while an item genuinely being overwritten (an eviction on a
full list) still maps here and detaches as before.

A branch with the fix and two regression tests is available — happy to open a PR.