strolling down the forest path

The sportsmen never saw a tiger, nor did they shoot anything else, though they saw number of deer, peacock, and wild board. They were giving up all hope of finding a tiger, and were beginning to shoot at jackals, when Grandfather, strolling down the forest path at some distance from the rest of the party, discovered a little tiger about eighteen inches long, hiding among the intricate roots of a banyan tree. Grandfather picked him up, and brought him home after the camp had broken up. He had the distinction of being the only member of the party to have bagged any game, dead or alive.


I got confused because of usage of `when' in the highlighted sentence between two[They were giving......... and Grandfather strolling down.....] different activities.

Could anyone assist me?

As the sportsmen were deciding that they would not find any animals, the writer's grandfather was walking along a path through the forest which was a little distance from where the rest of the people were gathered. He came across a tiger cub.

There are two events at the same time - the main party who are all together, talking about what to do, and Grandfather, who is walking down a path on his own.

It's not 'strolling':
They were giving...when grandfather discovered (the strolling part just describes what he was doing wehen he found it)

Again thanks to Anglika and Tdol.

But I can't understand, what you mean? --- Tdol.

Could you elaborarte your explanation?

'When' is used because it is a long action (past progressive) interrupted by a short one (past simple)