Group a line's runs in one home, part a line as its first fallback, and trim each portable line by its own runs

spec/flavour.md

@@ -564,12 +564,13 @@ marks array in order, outermost first: `_!adf:underline[x]_` gives marks `[em, u
 the array, not a set — and opens each spelling once over the longest run of adjacent inline nodes
 carrying an identical mark, attributes included, at that depth: `attrs: {}` differs from no `attrs`,
 and a directive spells it `{attrs=empty}`. A run breaks at every node the emitter carries, so no
-emitted carry sits inside a mark spelling. An `em`, `strike`, `strong` or `link` run also spans a
-node between two of its nodes when that node cannot hold the run's mark. It does so only where the
-spanned line reads back to the same nodes; otherwise the run breaks there. So `_One\` then `Two_`
-is one `em` run over two texts, the hard break between them unmarked, and
-``**Run `npm ci` first**`` is one `strong` run around unmarked code. `adfToPortableMarkdown` spans
-hard breaks and code spans the same way.
+emitted carry sits inside a mark spelling. An `em`, `strike`, `strong` or `link` run also crosses a
+node between two of its nodes when that node cannot hold the run's mark, unless the node holds an
+empty `marks`, which no spelling inside a mark holds. So `_One\` then `Two_` is one `em` run over two
+texts, the hard break between them unmarked, and ``**Run `npm ci` first**`` is one `strong` run
+around unmarked code. A line that needs any fallback, a carry or a dropped mark, breaks every run on
+it at those nodes instead. `adfToPortableMarkdown` crosses hard breaks and code spans the same way,
+and drops whitespace beside a hard break a run crosses, where CommonMark strips it.
 
 An inline node whose marks no nesting spells — a node holding a mark Atlassian's schema withholds
 from it, a mark type the first list above does not hold, an attrs key its spelling does not list, a value that is not the