If ‘Hide completed items’ is checked and one right-clicks on a folder and selects ‘Open selected files’, all files in the folder are opened. The expected behaviour would be to open just the incomplete files.

In D15284#321501, @huftis wrote:

If ‘Hide completed items’ is checked and one right-clicks on a folder and selects ‘Open selected files’, all files in the folder are opened. The expected behaviour would be to open just the incomplete files.

Yes, that was also the case for the other contextual menu items (search in files), selectedItems returned all items regardless of their filtered status.
I've changed the behavior so it only returns visible items.
https://phabricator.kde.org/R456:3216b30d5acec3e833cfefd4b0fe8a15bf6d0d33