QQuickText: fix fractional pointSize rounding error bug

Problem: The layout is initialized with a copy of the current font
before the laying out begins.
In cases when setupTextLayout() iterates through the main loop multiple
times, it will update the layout font, if it differs. (See code below)

if (!once) {
    if (pixelSize)
        scaledFont.setPixelSize(scaledFontSize);
    else
        scaledFont.setPointSizeF(scaledFontSize);
    if (layout.font() != scaledFont)
        layout.setFont(scaledFont);
}

The whole reason why we might update the font in the first place, is
because the QQuickText has a fontSizeMode property which can be set to
e.g. HorizontalFit, which will cause the layouting to downscale the font
in order to fit (assuming the text doesn't fit the space available),
instead of eliding.

The problem here is that QFont internally uses a float to store the point
size, and we would convert that value to an int when temporarily storing
it on the stack. This would modify the pointSize value in cases where
the pointSize is fractional, and cause a mismatch between the QQuickText
and QTextLayout.

The lineWidth is set only during the first loop iteration. During
successive iterations of the main loop, the layout's font would be updated
to one that has either a floored or ceiled pointSize.
If the pointSize is a fractional value, and is ceiled when being updated
during the second loop iteration, the layout would use a larger font
value, which would cause the QTextLayout::naturalTextWidth() to return a
larger value than that of the lineWidth, triggering eliding.

Solution: Keep the font precision, by storing the values as floats
instead of ints.

Fixes: QTBUG-92006
Pick-to: 6.2 6.3
Change-Id: Ibf64ac2dfbf262c6aae05b8eb8251d2f5a869b69
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

0 files changed, 0 insertions, 0 deletions