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Avoid reflow of shaped text #3566

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Oct 7, 2020
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Avoid reflow of shaped text #3566

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134 changes: 119 additions & 15 deletions src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/TextBlock.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1504,7 +1504,7 @@ protected sealed override Size ArrangeOverride(Size arrangeSize)
{
// Check if paragraph ellipsis are added to this line
bool ellipsis = ParagraphEllipsisShownOnLine(i, lineOffset.Y - contentOffset.Y);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, _textBlockCache._textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, _textBlockCache._textRunCache, ellipsis);

// Check that lineMetrics length and line length are in sync
// Workaround for (Crash when mouse over a Button with TextBlock). Re-enable this assert when MIL Text issue is fixed.
Expand Down Expand Up @@ -1638,7 +1638,7 @@ protected sealed override void OnRender(DrawingContext ctx)
{
using (line)
{
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, showParagraphEllipsis, lineProperties), lineMetrics.TextLineBreak, _textBlockCache._textRunCache, showParagraphEllipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, showParagraphEllipsis, lineProperties), lineMetrics.TextLineBreak, _textBlockCache._textRunCache, showParagraphEllipsis);

// Workaround for (Crash when mouse over a Button with TextBlock). Re-enable this assert when MIL Text issue is fixed.
//if (!showParagraphEllipsis)
Expand Down Expand Up @@ -1778,7 +1778,7 @@ protected virtual IInputElement InputHitTestCore(Point point)
{
// Check if paragraph ellipsis are rendered
bool ellipsis = ParagraphEllipsisShownOnLine(i, lineOffset);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);

// Verify consistency of line formatting
// Check that lineMetrics.Length is in sync with line.Length
Expand Down Expand Up @@ -1902,7 +1902,7 @@ protected virtual ReadOnlyCollection<Rect> GetRectanglesCore(ContentElement chil
{
// Check if paragraph ellipsis are rendered
bool ellipsis = ParagraphEllipsisShownOnLine(lineIndex, lineOffset);
line.Format(lineStart, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, lineStart, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);

// Verify consistency of line formatting
// Workaround for (Crash when mouse over a Button with TextBlock). Re-enable this assert when MIL Text issue is fixed.
Expand Down Expand Up @@ -2171,16 +2171,17 @@ internal void GetLineDetails(int dcp, int index, double lineVOffset, out int cch
double wrappingWidth = CalcWrappingWidth(RenderSize.Width);

TextRunCache textRunCache = new TextRunCache();
LineMetrics lineMetrics = GetLine(index);

// Retrieve details from the line.
using(Line line = CreateLine(lineProperties))
using (Line line = CreateLine(lineProperties))
{
// Format line. Set showParagraphEllipsis flag to false
TextLineBreak textLineBreak = GetLine(index).TextLineBreak;
bool ellipsis = ParagraphEllipsisShownOnLine(index, lineVOffset);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), textLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), textLineBreak, textRunCache, ellipsis);

MS.Internal.Invariant.Assert(GetLine(index).Length == line.Length, "Line length is out of sync");
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");

cchContent = line.ContentLength;
cchEllipses = line.GetEllipsesLength();
Expand Down Expand Up @@ -2219,15 +2220,16 @@ internal ITextPointer GetTextPositionFromDistance(int dcp, double distance, doub
lineVOffset -= contentOffset.Y;

TextRunCache textRunCache = new TextRunCache();
LineMetrics lineMetrics = GetLine(index);
ITextPointer pos;
using(Line line = CreateLine(lineProperties))
{
MS.Internal.Invariant.Assert(index >= 0 && index < LineCount);
TextLineBreak textLineBreak = GetLine(index).TextLineBreak;
bool ellipsis = ParagraphEllipsisShownOnLine(index, lineVOffset);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), textLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), textLineBreak, textRunCache, ellipsis);

MS.Internal.Invariant.Assert(GetLine(index).Length == line.Length, "Line length is out of sync");
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");

CharacterHit charIndex = line.GetTextPositionFromDistance(distance);
LogicalDirection logicalDirection;
Expand Down Expand Up @@ -2297,7 +2299,7 @@ internal Rect GetRectangleFromTextPosition(ITextPointer orientedPosition)
using(Line line = CreateLine(lineProperties))
{
bool ellipsis = ParagraphEllipsisShownOnLine(i, lineOffset);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);

// Check consistency of line length
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");
Expand Down Expand Up @@ -2399,7 +2401,7 @@ internal Geometry GetTightBoundingGeometryFromTextPositions(ITextPointer startPo
using (line)
{
bool ellipsis = ParagraphEllipsisShownOnLine(i, lineOffset);
line.Format(dcpLineStart, wrappingWidth, GetLineProperties(dcpLineStart == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcpLineStart, wrappingWidth, GetLineProperties(dcpLineStart == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);

if (Invariant.Strict)
{
Expand Down Expand Up @@ -2503,7 +2505,7 @@ internal bool IsAtCaretUnitBoundary(ITextPointer position, int dcp, int lineInde
{
// Format line. Set showParagraphEllipsis flag to false since we are not using information about
// ellipsis to change line offsets in this case.
line.Format(dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);

// Check consistency of line formatting
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");
Expand Down Expand Up @@ -2591,7 +2593,7 @@ internal ITextPointer GetNextCaretUnitPosition(ITextPointer position, LogicalDir
{
// Format line. Set showParagraphEllipsis flag to false since we are not using information about
// ellipsis to change line offsets in this case.
line.Format(dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);

// Check consistency of line formatting
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");
Expand Down Expand Up @@ -2700,7 +2702,7 @@ internal ITextPointer GetBackspaceCaretUnitPosition(ITextPointer position, int d
{
// Format line. Set showParagraphEllipsis flag to false since we are not using information about
// ellipsis to change line offsets in this case.
line.Format(dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(lineIndex == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, false);

// Check consistency of line formatting
MS.Internal.Invariant.Assert(lineMetrics.Length == line.Length, "Line length is out of sync");
Expand Down Expand Up @@ -3326,6 +3328,108 @@ private double CalcWrappingWidth(double width)
return width;
}

// ------------------------------------------------------------------
// Wrapper for line.Format that tries to make the same line-break decisions as Measure
// ------------------------------------------------------------------
private void Format(Line line, int length, int dcp, double wrappingWidth, TextParagraphProperties paragraphProperties, TextLineBreak textLineBreak, TextRunCache textRunCache, bool ellipsis)
{
line.Format(dcp, wrappingWidth, paragraphProperties, textLineBreak, textRunCache, ellipsis);

// line.Format can reflow (make a different line-break
// decision than it did during measure), contrary to the comment
// in CalcWrappingWidth "Reflowing will not happen when Width is
// between _previousDesiredSize.Width and ReferenceWidth", if the
// line contains text that gets shaped in a way that reduces the
// total width. Here is an example.
// Text="ABCDE IAATA Corp." TextWrapping=Wrap ReferenceWidth=115
// 1. Measure calls FormatLine(115), which determines that the full
// text is wider than 115 and breaks it after the second word.
// The resulting desired width is 83.3167 - the length of
// the first line "ABCDE IAATA"
// 2. Render, HitTest, et al. call FormatLine(83.3167), which determines
// that the first two words are already wider than 83.3167 and
// breaks after the first word.
// 3. FormatLine uses unshaped glyph widths to determine how much text
// to consider in line-breaking decisions. But it reports the
// width of the lines it produces using shaped glyph widths.
// In the example, the sequence "ATA" gets kerned closer together,
// making the shaped width of the first two words (83.3167)
// about 2.6 pixels less than the unshaped width (85.96).
// This is enough to produce the "reflowing".
// The consequences of reflowing are bad. In the example, the second
// word is not rendered, and programmatic editing crashes with FailFast.
//
// In light of this, we need to work harder to ensure that reflowing
// doesn't happen. The obvious idea to accomplish this is to change
// FormatLine to use shaped widths throughout, but that would mean
// changing the callbacks from LineServices and DWrite, and asserting
// that the changes have no unforseen consequences - out of scope.
// Instead, we'll call FormatLine with a target width large enough
// to produce the right line-break.
//
// This has consequences, especially when TextAlignment=Justify -
// the line is justified to the larger width rather than to wrappingWidth,
// which makes the text extend past the arrange-rect. To mitigate this,
// use the smallest width between wrappingWidth and ReferenceWidth that produces the
// right line-break.
//
// This fixes the cases of missing text and FailFast, at the cost of
// 1. more calls to FormatLine (perf hit)
// 2. justified text sticks out of the arrange-rect
// It's pay-for-play - we only do it on lines that reflow.

if (line.Length < length) // reflow happened
{
double goodWidth = _referenceSize.Width; // no reflow at this width
double badWidth = wrappingWidth; // reflow at this width
// Make sure that TextFormatter limitations are not exceeded.
TextDpi.EnsureValidLineWidth(ref goodWidth); // wrappingWidth is already valid, per CalcWrappingWidth

// The smallest good width can't be calcluated in advance, as it's
// dependent on the shaped and unshaped glyph-widths and the available
// width in a complicated way. Instead, binary search.
const double tolerance = 0.01; // allow a small overshoot, to limit the number of iterations

// In practice, the smallest good width is quite close to wrappingWidth,
// so start with "bottom-up binary search".
for (double delta = tolerance; /* goodWidth not found */; delta *= 2.0)
{
double width = badWidth + delta;
if (width > goodWidth)
break; // don't increase goodWidth

line.Format(dcp, width, paragraphProperties, textLineBreak, textRunCache, ellipsis);
if (line.Length < length)
{
badWidth = width;
}
else
{
goodWidth = width;
break;
}
}

// now do a regular binary search on the remaining interval
for (double delta = (goodWidth - badWidth) / 2.0; delta > tolerance; delta /= 2.0)
{
double width = badWidth + delta;
line.Format(dcp, width, paragraphProperties, textLineBreak, textRunCache, ellipsis);
if (line.Length < length)
{
badWidth = width;
}
else
{
goodWidth = width;
}
}

// now format at goodwidth, with no reflow
line.Format(dcp, goodWidth, paragraphProperties, textLineBreak, textRunCache, ellipsis);
}
}

// ------------------------------------------------------------------
// Aborts calculation by throwing exception if world has changed
// while in measure / arrange / render process.
Expand Down Expand Up @@ -3415,7 +3519,7 @@ private void AlignContent()
using (line)
{
bool ellipsis = ParagraphEllipsisShownOnLine(i, lineOffset);
line.Format(dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
Format(line, lineMetrics.Length, dcp, wrappingWidth, GetLineProperties(dcp == 0, lineProperties), lineMetrics.TextLineBreak, textRunCache, ellipsis);
double lineHeight = CalcLineAdvance(line.Height, lineProperties);

// Check consistency of line formatting
Expand Down