Merge pull request #7526 from dotCooCoo/perf/flatten-sweep-staggered-boxes

Flatten: boxes staggered right and down meet only the box before in the grouping sweep

docs/target-app-parity.md

@@ -1363,7 +1363,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   Object › Flatten Transparency takes a frame of threaded text out of the thread in the same way when Convert All Text to Outlines outlines it or when the frame becomes flat regions or an image, and the other frames keep the text they hold.
   Object › Flatten Transparency, the Flattener Preview, and EPS, PDF and print output that flatten transparency stop building a group's flat regions at the first region past the raster/vector balance's limit (10^(balance/20), rounded down) and rasterize the whole group without building the remaining regions. Building every region gives the same flattened art and the same files. With Clip Complex Regions on, a group builds every region and its image is clipped to them.
   Object › Flatten Transparency, the Flattener Preview, and EPS, PDF and print output that flatten transparency leave art without transparency as it is when it overlaps only other art without transparency, unless it overlaps a member of a see-through object's group and lies above that member and below the group's topmost object. Such art joins the group, and the group takes the place of its topmost object, so the stacking order stays the same. In files without transparency with Clip Complex Regions on, art under the group within one image pixel of a member joins it too. The 1 pt antialiasing margin goes around a group's art once, however deeply its groups nest. On a grid of 3,000 rectangles with a 0.5 pt stroke, 60 of them with a drop shadow, the Flattener Preview counts 180 affected objects where it counted 3,000, and Flatten Transparency with the Medium preset makes 240 flat regions and 2 images where it made one image of the whole grid. These rules are VectorCraft's own (`tests::flatten::opaque_art_that_touches_only_opaque_art_stays`, `tests::flatten::opaque_art_the_group_would_move_under_joins_it`).
-  When they group art for flattening, Object › Flatten Transparency, the Flattener Preview, and EPS, PDF and print output that flatten transparency compare the box of an object without transparency only with the boxes of see-through objects in the pass that splits art by transparency. In both passes, a box that lies inside an earlier box that ends no further right and joins everything it overlaps is not compared with later boxes. With one see-through object inside 512 nested objects without transparency, the grouping compares 1,024 pairs of boxes where it compared 262,145, and the groups stay the same (`cmd::flatten::analysis::tests::nested_opaque_boxes_meet_only_the_see_through_box`, `a_shrinking_nest_meets_only_its_outermost_box`).
+  When they group art for flattening, Object › Flatten Transparency, the Flattener Preview, and EPS, PDF and print output that flatten transparency compare the box of an object without transparency only with the boxes of see-through objects in the pass that splits art by transparency. In both passes, a box that lies inside an earlier box that ends no further right and joins everything it overlaps is not compared with later boxes. With one see-through object inside 512 nested objects without transparency, the grouping compares 1,024 pairs of boxes where it compared 262,145, and the groups stay the same (`cmd::flatten::analysis::tests::nested_opaque_boxes_meet_only_the_see_through_box`, `a_shrinking_nest_meets_only_its_outermost_box`). When a later box that joins everything it overlaps covers an earlier box to the right and below, the grouping compares the earlier box with still later boxes only above the later box's top: 512 boxes, each 1 pt right of and below the one before, compare 511 pairs where they compared 130,816, with and without transparency, and the groups stay the same (`cmd::flatten::analysis::tests::staggered_boxes_meet_only_the_box_before`, `staggered_boxes_group_as_comparing_every_pair`).
   New Symbol and Redefine Symbol on a frame of threaded text take it out of the thread with the text it shows, which the symbol's art holds, and the other frames keep the text they hold. When the last frame goes into the symbol, the text it has no room for goes after the text of the last frame that stays. When every frame of a thread goes into the symbol, the copy of the last frame keeps all its text.
   With Optical Margin Alignment on, or Roman Hanging Punctuation on in the first paragraph, the glyphs at the ends of left-to-right type on a path hang outside its brackets, measured along the path. The type hangs at the start bracket when its line is placed there, hangs at the end bracket when the line is placed there, and hangs at both when it is centered. Glyphs that hang past the end bracket do not make the type overflow, and a hang stops at an open path's end. SVG export writes such type as outlines.
   Type › Type on a Path › Center on Top and Center on Bottom (`type.pathCenter.top`, `type.pathCenter.bottom`, also on the Contextual Task Bar for type on a closed path) slide type on a closed path so the middle of its span lands on the path's topmost or bottommost point as drawn, keep the span's length, and set the first paragraph to Center unless it is Center or Justify Center. This is VectorCraft's own behavior (`crates/geom/src/arc.rs`, `crates/engine/src/tests/pathtype.rs`).