Dense infill under solid layer
Someone is working on PrusaSlicer:
if the infill% is under 40%, it search a supported area under each solid surface, with only straight lines between perimeters. When it find one, it use a special infill process that lay a perimeter (with overlap, so it's ~supported by the current internal perimeter and then a 42% rectilinear infill inside it. This will prevent any curling that may ruin your top surface, and ensure everything can be supported even with an infill ratio of 0%
note: beta feature, not yet in the release package. It will replace the manual dense infill.
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This interface can have lower infill density (50 - 90 %) so the bridges can interleave and the same infill type can be used (most of types cannot reach 100 %).