P: Black border in Filmstrip after installing 15.3

Same here, I have upvoted your post,
Mine only appears when using AI tools like Denoise and Super Resolution

Have upvoted; same for me on Windows 10 running Lightroom Classix 15.3.

I am on a MS Windows machine too.  And I am also experiencing the same issue with the black box. It goes away if I exit out of Lightroom Classic and enter back in.  But one thing I noticed it is also doing is once a photo has a black outline over it, it also removes the white “Pick” flag that I previously entered when I chose the photo as a picked selection.  

Interestingly, I reverted back to the previous version of 15.2.1, the black box no longer seems to appear but I noticed that the pick flag is still gone if had the black box around the image in the 15.3 version. I was easily able to add them pick flag back in, but I am not sure if it removed other pick flags I have on other photos in my LR catalog.

Hope that this is fixed soon. In the meantime, I decided to stay in version 15.2.1 until it is fixed. 

I have the same problem. :(

It's a bug. It was definitely not like this before.

I'm losing valuable real estate in my filmstrip because of that.

Another bug introduced in 15.3: the point where you zoom in when switching between photos now resets from photo to photo, but not always (kind of random behavior)

Adobe, please fix.

Using Win 11 Pro + LrC 15.3

same for me in  15.3, updated yesterday.  I don’t use the AI Denoise (only the manual NR) and it happens to me.  Restarting clears it but then appears after a few minutes.  Can’t determine what triggers it.

If you close LrC and re-open it, the black border has gone. It happened when I updated AI tools after some new edits.

This is not normal. There is a bug here in LrC 15.3.

In the Filmstrip, the selected photos has always had a white outer border around the thumbnail. All thumbnails also have a thin black border. As  @Bob Somrak points out, if a photo has the Pick flag set, the thin inner border changes to white, making the border on selected photos with the Pick flag appear to have a thicker white border.

Below is a screen grab of the Filmstrip showing this behaviour in LrC 15.2.1 after photo has been Denoised:

The appearance of the Filmstrip thumbnail borders didn't change when Denoise was applied.

On launching LrC 15.3, the Filmstrip thumbnails initially show the same borders as they do in LrC 15.2.1, as shown below:

When Denoise is applied to photo 6, the thin inner border thickens. As you hover over other thumbnails in the Filmstrip, their thin inner borders also become thicker. In the screen grab below, I had hovered over photos 1 and 8:

Note the difference between photo 8 and photo 3, both with the Pick flag, but photo 3 hasn't been hovered over, so has a thin white border. Also note the thumbnails right of those with thicker borders have a thicker left border.

Switching to the Library results in all thumbnails in the Filmstrip being shown with thicker inner borders, as shown here:

None of this border thickening happened before 15.3. When LrC 15.3 is closed and relaunched, all Filmstrip thumbnail borders return to the normal thickness until the next Enhance (Denoise, Raw Details or Super Resolution).

It’s a regressive bug in LrC 15.3 and shouldn’t happen.

this happens everytime i use denoise on a photo borders aper on all other photos in a strip, definetly not happening version prior to this

Weirdly I am seeing opposite behavior. This is unpicked:

This is picked:

So you get a thinner black line with the pick flag on.

This is with HiDPI on unpicked:

This is HiDPI picked flag on:

Which interestingly shows three different shades. A light grey, a white, and a black line.