How to stop roof water from overflowing and rotting the window sill

What people tried

Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.

1. 1
   Adding a gutter to the upper section or roof edge to redirect water flow
2. 2
   Installing flashing at the roof edge
3. 3
   Applying a wood hardener or penetrating epoxy to the sill
4. 4
   Replacing the rotted sill and trim after proper re-flashing
5. 5
   Installing kick-out flashing under the shingles or cladding
6. 6
   Adding a small splash guard on the gutter
7. 7
   Applying silicone on an angle to divert water

In their words

Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.