Zoom blurry in full screen when zooming with mouse, not Z or Spacebar

[Pavel5E74]

When I'm in develop module editing RAW photos, I switch to full screen with F key, then I click with left mouse into the image which zooms the image to the selected zoom ratio, the result is a zoomed view that's slightly blurry. If I then grab the image with my left mouse and move around, it immediately sharpens up. If I then left click again the image zooms out and is again slightly blurry. 

If exit out of full screen the image immediately sharpens up.

If I choose to zoom with the spacebar key or the Z key, everything works correctly. Zoomed in picture is sharp.

The problem occurs only when in full screen view.

The problem occurs both with 50% and 100% zoom.

This is repeatable on every RAW photo.

The problem is present with GPU accelleration both on or off.

The problem doesn't occur with JPG images.

The problem doesn't occur in Library view in full screen.

I'm using Windows 11 24H2 and Lightroom Classic 14.1.1. CPU Intel 14700K, GPU Nvidia RTX4090.

[GoldingD]

Can you post in a reply a screenshot of your LrC /Preferences/Performance/

[Pavel5E74]

[GoldingD]

I do not have a clue.

[drtonyb]

This is a bug in LrC with the generation of poor quality Standard Previews.

I see it all the time because I know how to produce it.

Here's an example:

I created a new catalog for this test and in Edit > Catalog Settings, I set Standard Preview Size to 2048 pixels. I Imported one photo with Build Previews set to Minimal.

I then switched to Develop and created a Virtual Copy (in Develop), then Cropped it to a size a bit wider than my monitor width, which is 1920 pixels.

While in Develop, disk-based previews are not generated; it uses memory cached previews for speed. Here is what I see in Develop after my crop:

The image is sharp and if I use the F key to show a Full Screen Preview, it is also sharp.

When I switch to Library Loupe, the image looks blurred:

It also appears blurred if I press F to view it in Full Screen Preview. My crop exaggerates this problem. If I click to Zoom in to 100% (or anything other than Fit), LrC generates 1:1 Previews for the photo and it then appears sharp. This 1:1 Preview only lasts until you make another edit adjusment and a poor quality Standard Preview will replace it. If you have zoomed to get a 1:1 Preview generated in Library Loupe, you can see the Standard Preview replace it in real time by bumping say the Clarity in the Library's Quick Develop panel; the photo changes to the poor quality again.

Until fixed, the work around is to Zoom in while in Library Loupe to force the generation of a 1:1 Preview. Alternatively, I have found that setting the Preview Size to 2880 pixels seems to avoid this problem.