Many current Linux distributions like Linux Mint 18 come with KDE 5 as X-window, which is still quiet buggy. Beyond Compare has not been adapted to KDE 5 with its default file browser Dolphin yet. I fixed this by the following:
As I don't use the Konqueror browser and nothing else was mentioned about Dolphin, I installed the bcompare-ext-kde library and its desktop file. With 'echo $KDE_SESSION_VERSION' I checked that KDE 5 is installed. Within my $HOME folder below .local/share/ I had to add the subfolders kservices5/ServiceMenus. I had to adapt the script /usr/lib/beyondcompare/kde_context_menu:
In line 65 and 417 replaced
< if [ "$KDE_SESSION_VERSION" = "4" ]; then
--- by
> if [ "$KDE_SESSION_VERSION" = "5" ]; then
In line 419 added new line
> kbuildsycoca5 &> /dev/null
In line 72 - 80 replaced "$KDEBASE/share/kde4/services/" by "$HOME/.local/share/kservices5/"
After a reboot there should be two files (beyondcompare_selectdir.desktop, beyondcompare_selectfile.desktop) below $HOME/.local/share/kservices5/ServiceMenus/. If not, they can be copied from $HOME/.kde/share/kde4/services/ServiceMenus/. The services settings of Dolphin must be checked for "Select Left File" and "Compare to ..." to be selected.