Affected versions

• 96.0a1 (20211202215058)
• 95.0 (20211129150630)
• 94.0.2 (20211119140621)
• 91.3.0esr (20211028170545)

Affected platforms

• Windows10 x64
• macOS 10.15

Steps to reproduce

1. Open Firefox and the import wizard window.
2. Change between browsers and observe the Next button.

Expected result

• Next button should not change.

Actual result

• String from the Next button flashes when changing browsers.

Regression range

• Last good revision: 49caadff1f4f125234fb16f88b02e4cd5e558322
  First bad revision: cc33a0848d854a8ed56a1e0a64897924757ae21d
  Pushlog:
  https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=49caadff1f4f125234fb16f88b02e4cd5e558322&tochange=cc33a0848d854a8ed56a1e0a64897924757ae21d

Notes

• Screen recording attached.
• With Light theme, focus flashes when changing browsers.
• This is sometimes intermittent while using the mouse, but with the keyboard is more reproducible.

The code kind of asks for this:

    this._wiz.canAdvance = false;
    this._wiz.canRewind = false;
    let result = MigrationUtils.spinResolve(promise);
    this._wiz.canAdvance = canAdvance;
    this._wiz.canRewind = canRewind;

It disables the button, spins the event loop, and re-enables the button again.

In the past, painting was too sluggish to happen during the event loop spin. Now it happens instantly.