The "The Tails documentation launcher on the desktop works…" scenarios are fragile

It breaks very often on Jenkins:

The Dogtail script raised: SearchError: child of [desktop frame | main]: "tails-documentation" application (RuntimeError)
./features/support/helpers/dogtail.rb:81:in `run'
./features/support/helpers/dogtail.rb:70:in `initialize'
./features/step_definitions/checks.rb:113:in `/^the documentation viewer opens the "(Support|Getting started)" page$/'
features/documentation.feature:7:in `Then the documentation viewer opens the "Getting started" page'

The screenshot shows that the doc desktop icon is selected but the documentation viewer is nowhere to be seen.

Parent Task: #10288

Related issues

Related to #15321 (closed)
Related to #11717 (closed)
Related to #16283 (closed)

Original created by @intrigeri on 15514 (Redmine)

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