Steps to reproduce

This is a low-frequency crash observed in production via Crashlytics. It occurs while dragging a text selection handle when the underlying RenderEditable is momentarily in a degenerate layout state (zero preferred line height, or a non-invertible transform that makes globalToLocal return non-finite coordinates).

In our data it is concentrated on foldable devices (Samsung Galaxy Z Flip 5 / SM-F731N), which strongly suggests the trigger is a layout transition (fold/unfold) happening mid-drag of a selection handle, transiently producing preferredLineHeight == 0 and/or a degenerate transform.

We do not have a deterministic minimal repro (it's a timing/layout race), but the root cause is unambiguous from the source — see below.

Expected results

Dragging a selection handle while the editable is in a transient degenerate layout state should be a no-op (or clamp), not throw.

Actual results

Fatal (uncaught-in-callback) exception:

Unsupported operation: Infinity or NaN toInt

reported through GestureRecognizer.invokeCallback → FlutterError.reportError.

Stack trace (symbolicated):

TextSelectionOverlay._getHandleDy (package:flutter/src/widgets/text_selection.dart)
TextSelectionOverlay._handleSelectionStartHandleDragUpdate (text_selection.dart:915)
SelectionOverlay._handleStartHandleDragUpdate (text_selection.dart:1296)
DragGestureRecognizer._checkUpdate.<fn> (monodrag.dart:871)
GestureRecognizer.invokeCallback (recognizer.dart:345)
DragGestureRecognizer._checkUpdate (monodrag.dart:871)
DragGestureRecognizer.handleEvent (monodrag.dart:713)
PointerRouter._dispatch (pointer_router.dart:97)
...

(The same applies to _handleSelectionEndHandleDragUpdate.)

Root cause

_getHandleDy in packages/flutter/lib/src/widgets/text_selection.dart (unchanged on master as of June 2026):

double _getHandleDy(double dragDy, double handleDy) {
  final double distanceDragged = dragDy - handleDy;
  final int dragDirection = distanceDragged < 0.0 ? -1 : 1;
  final int linesDragged =
      dragDirection * (distanceDragged.abs() / renderObject.preferredLineHeight).floor();
  return handleDy + linesDragged * renderObject.preferredLineHeight;
}

(distanceDragged.abs() / renderObject.preferredLineHeight) becomes:

• Infinity when renderObject.preferredLineHeight == 0 (division by zero with a positive numerator), or
• NaN when distanceDragged is non-finite — which happens when renderObject.globalToLocal(...) is fed through a non-invertible transform (a degenerate/zero-size render object during a layout transition), producing NaN/Infinity coordinates at the two call sites (text_selection.dart:782-785 and :915-918).

.floor() returns int, so on a non-finite double it throws UnsupportedError('Infinity or NaN toInt').

Proposed fix

Guard against a non-finite result before calling .floor():

double _getHandleDy(double dragDy, double handleDy) {
  final double distanceDragged = dragDy - handleDy;
  final double lineHeight = renderObject.preferredLineHeight;
  // A degenerate layout (e.g. a foldable mid fold/unfold) can momentarily make
  // preferredLineHeight 0, or make globalToLocal return non-finite coordinates.
  // Without this guard the division below is Infinity/NaN and .floor() throws
  // "Unsupported operation: Infinity or NaN toInt".
  if (lineHeight <= 0.0 || !distanceDragged.isFinite) {
    return handleDy;
  }
  final int dragDirection = distanceDragged < 0.0 ? -1 : 1;
  final int linesDragged = dragDirection * (distanceDragged.abs() / lineHeight).floor();
  return handleDy + linesDragged * lineHeight;
}

I'm happy to send a PR with this guard if it's welcome.

Flutter / Dart version

Flutter 3.38.9 • channel stable
Framework • revision 67323de285 • 2026-01-28
Engine • hash 5eb06b7ad5bb8cbc22c5230264c7a00ceac7674b (revision 587c18f873)
Dart SDK 3.10.x

Affected platform: Android (observed on Samsung Galaxy Z Flip 5 / SM-F731N). The vulnerable code is platform-agnostic; the path is just easiest to hit on foldables.

Confirmed the same unguarded _getHandleDy is present on master, so this is not fixed in any release after 3.38.9.