Argh - popup window when using keyboard-x to reject photos

I can't duplicate this behavior either.  I can set the filter to Unflagged and hold the X key down or tap it quickly and is works correctly.  

Also, upload the screen recording to Youtube or Dropbox and including the sharing link in your bug report.

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That's strange. I'm not able to reprodcue that behavior. I suggest you make a full-resolution screen recording of your entire LR window showing the bad behavior and submit a bug report to the official Adobe feedback forum:

https://feedback.photoshop.com/photoshop_family/categories/photoshop_family_photoshop_lightroom

Be sure to include the first ten lines of the menu command Help > System Info.  Provide all the details, rather than linking back to this thread -- experience shows that Adobe developers tend not to follow the links.

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I can't reproduce that behavior and I don't recall seeing other reports about this problem. Initial troubleshooting steps:

1. Attachments are currently broken in this forum and we can't see "screenshot_33.png".  Can you please reply and insert the screenshot inline into the post using the picture icon:

2. Please copy here the first ten lines from the menu command Help > System Info, so we know precisely which version and platform you're on.

3. Is the menu item Photo > Auto Advance checked or unchecked?

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Please Edit your post and remove the screen shot you attached. the forum is broken and no one can download attachments.

Use the Proper method for including a screen shot by using the Insert Photos icon in the tool bar to place the image or images you want to show in line with the text of you post.