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Add an adjustment to currentLineWidth comparisons when pushing greedy line breaks #21356

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jason-simmons merged 1 commit into from
Oct 7, 2020
Merged

Add an adjustment to currentLineWidth comparisons when pushing greedy line breaks #21356

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Add an adjustment to currentLineWidth comparisons when pushing greedy… 
… line breaks

This is similar to the workaround used for
flutter/flutter#30347

The Minikin line breaker inserts greedy breaks based on a comparison of
postBreak width and currentLineWidth.  currentLineWidth is provided by
the framework based on previous calls to Layout::measureText.
That calculation may not exactly match the calculation of postBreak.

This change ensures that breaks are only added if the difference
between postBreak and currentLineWidth is significant.

Fixes flutter/flutter#65419
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@jason-simmons
jason-simmons committed Sep 23, 2020
commit 1b203c65109932d23d940329f8f74b6b62b392e6
13 changes: 10 additions & 3 deletions third_party/txt/src/minikin/LineBreaker.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,12 @@ const size_t MAX_TEXT_BUF_RETAIN = 32678;
// Maximum amount that spaces can shrink, in justified text.
const float SHRINKABILITY = 1.0 / 3.0;

// libtxt: Add a fudge factor to comparisons between currentLineWidth and
// postBreak width. The currentLineWidth passed by the Flutter framework
// is based on maxIntrinsicWidth/Layout::measureText calculations that may
// not precisely match the postBreak width.
const float LIBTXT_WIDTH_ADJUST = 0.00001;

void LineBreaker::setLocale(const icu::Locale& locale, Hyphenator* hyphenator) {
mWordBreaker.setLocale(locale);
mLocale = locale;
Expand Down Expand Up @@ -240,7 +246,7 @@ void LineBreaker::addWordBreak(size_t offset,
// libtxt: add a fudge factor to this comparison. The currentLineWidth passed
// by the framework is based on maxIntrinsicWidth/Layout::measureText
// calculations that may not precisely match the postBreak width.
if (postBreak - width > currentLineWidth() + 0.00001) {
if (postBreak - width > currentLineWidth() + LIBTXT_WIDTH_ADJUST) {
// Add desperate breaks.
// Note: these breaks are based on the shaping of the (non-broken) original
// text; they are imprecise especially in the presence of kerning,
Expand Down Expand Up @@ -305,7 +311,7 @@ void LineBreaker::addCandidate(Candidate cand) {
// mCandidates, and mPreBreak is its preBreak value. mBestBreak is the index
// of the best line breaking candidate we have found since then, and
// mBestScore is its penalty.
if (cand.postBreak - mPreBreak > currentLineWidth()) {
if (cand.postBreak - mPreBreak > currentLineWidth() + LIBTXT_WIDTH_ADJUST) {
// This break would create an overfull line, pick the best break and break
// there (greedy)
if (mBestBreak == mLastBreak) {
Expand All @@ -316,7 +322,8 @@ void LineBreaker::addCandidate(Candidate cand) {
}

while (mLastBreak != candIndex &&
cand.postBreak - mPreBreak > currentLineWidth()) {
cand.postBreak - mPreBreak >
currentLineWidth() + LIBTXT_WIDTH_ADJUST) {
// We should rarely come here. But if we are here, we have broken the line,
// but the remaining part still doesn't fit. We now need to break at the
// second best place after the last break, but we have not kept that
Expand Down