This test appears to pass on master even without your change.

Improved this test. Now it fails on master, as when the image is changed to an uncached image, the placeholder isn't shown until the animation starts.

Is all this necessary? I naively expected that we would just need an instance variable (e.g. targetLoaded = false) and set that to true the first time frame != null. Then on line 443, we just pass targetLoaded to isTargetLoaded.

Maybe I am missing something, though?

1. If you make the above-suggested change, run the sample code (attached in the issue), and you change the image (by clicking) a few times, after 4 changes (clicks) the view goes blank, and nothing will happen after any further clicks. This is because, after 3 clicks, the image changes to a loaded image, setting wasSynchronouslyLoaded to true, disposing _AnimatedFadeOutFadeIn and the animations. Now when the 4th click occurs, _AnimatedFadeOutFadeIn is rebuilt, resetting the opacity animations. This can be fixed by removing the wasSynchronouslyLoaded if statement, so that _AnimatedFadeOutFadeIn is never disposed.
2. IMO the _AnimatedFadeOutFadeIn widget should be added to the tree only when the actual fade-out-fade-in is required, and removed when not. The above change (along with point 1) will cause it to exist on the tree forever.

It would be more performant by keeping the subtree the same, otherwise, they may be built from scratch

I can't reproduce the issue you mention after 4 clicks. This is the code i use

frameBuilder: (BuildContext context, Widget child, int? frame, bool wasSynchronouslyLoaded) {
        if (wasSynchronouslyLoaded) {
          _oldImage = child;
          return child;
        }
        if (frame != null) {
          // This is the first image.
          _oldImage = child;
        }
        return _AnimatedFadeOutFadeIn(
          target: child,
          targetProxyAnimation: _imageAnimation,
          placeholder: _image(
            image: widget.placeholder,
            errorBuilder: widget.placeholderErrorBuilder,
            opacity: _placeholderAnimation,
            fit: widget.placeholderFit ?? widget.fit,
          ),
          placeholderProxyAnimation: _placeholderAnimation,
          isTargetLoaded: _oldImage != null,
          fadeInDuration: widget.fadeInDuration,
          fadeOutDuration: widget.fadeOutDuration,
          fadeInCurve: widget.fadeInCurve,
          fadeOutCurve: widget.fadeOutCurve,
        );
      },

The issue with the current code is that after the image is faded in, it will cause a an unnecessary rebuild to dispose _AnimatedFadeOutFadeIn and rebuild the image widget which is not ideal

i am able to reproduce it on my side with your code.

The issue with the current code is that after the image is faded in, it will cause a an unnecessary rebuild to dispose _AnimatedFadeOutFadeIn and rebuild the image widget which is not ideal.

this can be fixed if we remove the wasSynchronouslyLoaded if block and let the _AnimatedFadeOutFadeIn return the image in it.

disposing _AnimatedFadeOutFadeIn and the animations. Now when the 4th click occurs, _AnimatedFadeOutFadeIn is rebuilt, resetting the opacity animations.

Wheere does the animations is disposed? It looks like we do need to reset the animation when the wasSynchronouslyLoaded is true to prepare for future rebuilds?

Where does the animations is disposed?

my bad, animations are not disposed, just the widget.

It looks like we do need to reset the animation when the wasSynchronouslyLoaded is true to prepare for future rebuilds?

As per gapless playback, there won't be any future animations. The old image will just be replaced by the new one after it is loaded.

why remove this logic?

as the comment suggests, this code jumps back to the placeholder image for the "new" target image.

i.e., old image --[user changes the image]--> jump to placeholder --[wait for new image]--> fade in new image.

but according to image gapless playback, the placeholder will never be shown again, the old image will act as a placeholder and the new image replace the old one w/o "fade in fade out".