Created attachment 185307 [details]
Video showing the issue

SUMMARY
The clipboard somehow interferes with (gtk) entry fields, resulting in premature/unprompted acceptance of gtk autocomplete suggestions. I can reliably reproduce this in multiple applications (e.g. GTK file open dialog, nemo file manager path input field). Erasing the clipboard history or disabling it entirely makes the issue go away

STEPS TO REPRODUCE
1. Enable the clipboard thingy, and copy some text.
2. Open a GTK application that has a field which supports auto completion. I use nemo in the video, you could also use the GTK file chooser dialog (Ctrl+L to enter a path).
3. Start typing a file path, which does exist, and has at least a couple characters unique compared to other files in the same directory (so that the inline completion will trigger).
4. Keep typing in the path, at some point the inline completion suggestion will show up. It will fill in the rest of the path for you, but the part it filled in will be selected, so that any new characters you type replace the selection.
5. Continue typing the path without accepting the suggestion using the Tab key.
6. At some point, the suggestion will be accepted for you, the cursor moves forward and the text is unselected.

If it doesn't immediately occur, try it again a couple times. Try giving it a bit of speed too.

7. Erase the entire clipboard history, and try the whole process again. The problem goes away and the suggestion won't be accepted as long as you don't press the Tab key.

* See also the video

IMPACT
You try to type a path like "~/.local". Midway through typing it the inline suggestion is accepted against your wishes, and you end up with "~/.local/al".

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.18.0
Qt Version: 6.9.2