Issue #3273317 by clayfreeman, larowlan, tim.plunkett, danflanagan8: Add 'in_preview' variable to block and layout templates

Issue #2938132 by tedbow, tim.plunkett, xjm, EclipseGc, AaronMcHale, alexpott, DyanneNova: Ship layouts that make sense with Layout Builder's concept of sections

Issue #2834025 by tim.plunkett, Manuel Garcia, swentel, japerry: Mark Layout Discovery as a stable module

Issue #2885877 by tim.plunkett, EclipseGc: Add support for per-region attributes to Layout Templates

Issue #2852608 by Manuel Garcia, DyanneNova, markconroy, tim.plunkett, joelpittet, DickJohnson, Scorpid: Review layout CSS and markup

Issue #2857410 by tim.plunkett, DamienMcKenna: Adding a page-level #prefix or #suffix causes it to be output twice on one-col layout

Issue #2840832 by tim.plunkett, DickJohnson, yoroy, DyanneNova, webchick: Add Panels layouts to core