Created attachment 87884 [details]
The offending PDF
IMPORTANT NOTE: for this PDF to be even displayed, one should first fix #70671 (I propose a simple fix therein).
In the attached PDF, the blue line should be displayed when behind the yellow line. The yellow line is the last element of a knockout transparency group which includes another group containing the blue line. The transparency of the yellow line should be based on the original backdrop of the group (i.e., the empty page).
The problem is caused by the restoration of the graphic state in Gfx::drawForm (Gfx.cc:4947). It calls CairoOutputDev::restoreState which, instead of letting cairo restore the current operator, sets it using CairoOutputDev::updateBlendMode. Hence while the first elements of a knockout groups are drawn using the right operator, i.e., CAIRO_OPERATOR_SOURCE, after a nested group appears, the operator is reset (most probably to CAIRO_OPERATOR_OVER, corresponding to Normal blend mode).
I fixed this in my local repo by moving the call to updateBlendMode from restoreState to paintTransparencyGroup. I am not sure this is the perfect fix, but it does work for my test case.
Sorry, I should proofread myself a little more. "the blue line should be displayed" is to be read "the blue line should not be displayed" (see forthcoming screenshots).
Created attachment 87885 [details]
Created attachment 87886 [details]