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d74c9378b8
The previous paragraph layout algorithm had a couple of flaws: - It always produced line break opportunities between runs although on the textual level there might have been none. - It didn't handle trailing spacing correctly in some cases. - It wouldn't have been easily adaptable to Knuth-Plass style optimal line breaking because it was fundamentally structured first-fit run-by-run. The new paragraph layout algorithm fixes these flaws. It proceeds roughly in the following stages: 1. Collect all text in the paragraph. 2. Compute BiDi embedding levels. 3. Shape all runs, layout all children and store the resulting items in a reusable (possibly even cacheable) `ParLayout`. 3. Iterate over all line breaks in the concatenated text. 4. Construct lightweight `LineLayout` objects for full lines instead of runs. These mostly borrow from the `ParLayout` and only reshape the first and last run if necessary. The design allows to use Harfbuzz's UNSAFE_TO_BREAK mechanism to make reshaping more efficient. The size of a `LineLayout` can be measured without building the line's frame. 5. Build only the selected line's frames and stack them.