The rules for typesetting math differ from the rules for typesetting
text. For example, two italic 'f' chars have to be typeset more closely
than two 'o' chars in text mode, but not in math mode. Qt provides a
method that returns the distance appropriate for drawing a subsequent
character in text mode, but nothing for math mode. Typically, the
distance appropriate for drawing the next character in math mode is
the actual width span by the character, corrected by the rules of
an appendix in the TeXbook. Recently, those rules are followed more
closely in LyX but not exactly, and we have to find a way to adapt to them.
Some symbols may need more spacing around them than the width they span.
So, we use the distance suggested by Qt, unless it is less than the
width of the rectangle bounding the symbol. Before Qt 5.11 the used method
was QFontMetrics::width(), but since then it has been declared obsolete
in favor of QFontMetrics::horizontalAdvance(), whose name conveys better
its meaning.
Both QTextLine::naturalTextWidth() and QTextLine::horizontalAdvance()
return the same value for \fint. However, examining esint10.ttf with
fontforge does not reveal any issue with the metrics. The fact that
\fint seems to be the only affected symbol might be due to its code
point, which corresponds to a space, so that maybe Qt makes some
assumptions on the metrics.
As QTextLine::naturalTextWidth() returns the width of the line that is
occupied by text, in the case of a single symbol we can obtain the
same value by using the width of the rectangle bounding the symbol.
Since be836909c52 the positioning of super- and subscripts
for symbol fonts has been broken because the metrics of the
font of the environment (rather than those of the symbol itself)
were used.
There are two techniques that I know of for forcing the direction of a
string, regardlessly of whether its contents is naturally LtR, RtL or
undecided.
1/ The unicode LTR/LTR override characters. This is supposed to be the
clean way, however, it does not seem to work with Qt 5.14 (see
#11691).
2/ The undocumented QTextLayout::setFlags method. This is used
internally and allows to pass the (undocumented) flags
Qt::TextForceRightToLeft and Qt::TextForceLeftToRight. This was
used until we had issues with Qt 5.11 (see #11284).
In order to get the best of both worlds, this patch allows to enable
those two methods separately, and actually enables both at the same
time by default!
(hopefully) Fixes bug #11691.
In particular, the directory frontends/qt4 is renamed to frontends/qt.
Many configurations file have to be updated. All mentions of qt4 in
the source have been audited, and changed to qt if necessary.
The only part that has not been updated is the CMake build system.
