  One of the most enduring and intensively studied problems of X-ray astronomy
is the disagreement of state-of-the art theory and observations for the
intensity ratio of two Fe XVII transitions of crucial value for plasma
diagnostics, dubbed 3C and 3D. We unravel this conundrum at the PETRA III
synchrotron facility by increasing the resolving power two and a half times and
the signal-to-noise ratio thousand-fold compared to our previous work. The
Lorentzian wings had hitherto been indistinguishable from the background and
were thus not modeled, resulting in a biased line-strength estimation. The
present experimental oscillator-strength ratio
$R_\mathrm{exp}=f_{\mathrm{3C}}/f_{\mathrm{3D}}=3.51(2)_{\mathrm{stat}}(7)_{\mathrm{sys}}$
agrees with our state-of-the-art calculation of $R_\mathrm{th}=3.55(2)$, as
well as with some previous theoretical predictions. To further rule out any
uncertainties associated with the measured ratio, we also determined the
individual natural linewidths and oscillator strengths of 3C and 3D
transitions, which also agree well with the theory. This finally resolves the
decades-old mystery of Fe XVII oscillator strengths.
