Android A11Y: Fix value rounding

QAccessibleInterface stores values as QVariants. When dealing with
double values, as a result of some calculation (for example, Slider
value update), rounding errors can be introduced.
When converting double values to QString using QVariant::toString(),
these rounding errors result in strings like
0.30000000000000004 instead of 0.3 or
2.7755575615628914e-17 instead of 0.3 - 3 * 0.1 and similar zeroes.

To fix this issue, this patch introduces a custom conversion for
floating-point values. The idea is to convert QVariant to double,
and then convert double to QString using 'f' format and a suitable
precision, determined from the UI element's minimumStepSize(), if
it has one, otherwise falling back to QString::number()'s default
(which is 6).

Task-number: QTBUG-93396
Pick-to: 6.3 6.2 5.15
Change-Id: Ia5ca7345812e39629e9c191b6d8b896a8f51de80
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

src/plugins/platforms/android/androidjniaccessibility.cpp

@@ -358,8 +358,50 @@ if (!clazz) { \
         QString valueStr;
         QAccessibleValueInterface *valueIface = iface->valueInterface();
         if (valueIface) {
-            // TODO: fix double-to-string conversion
-            valueStr = valueIface->currentValue().toString();
+            const QVariant valueVar = valueIface->currentValue();
+            const auto type = valueVar.typeId();
+            if (type == QMetaType::Double || type == QMetaType::Float) {
+                // QVariant's toString() formats floating-point values with
+                // FloatingPointShortest, which is not an accessible
+                // representation; nor, in many cases, is it suitable to the UI
+                // element whose value we're looking at. So roll our own
+                // A11Y-friendly conversion to string.
+                const double val = valueVar.toDouble();
+                // Try to use minimumStepSize() to determine precision
+                bool stepIsValid = false;
+                const double step = qAbs(valueIface->minimumStepSize().toDouble(&stepIsValid));
+                if (!stepIsValid || qFuzzyIsNull(step)) {
+                    // Ignore step, use default precision
+                    valueStr = qFuzzyIsNull(val) ? u"0"_qs : QString::number(val, 'f');
+                } else {
+                    const int precision = [](double s) {
+                        int count = 0;
+                        while (s < 1. && !qFuzzyCompare(s, 1.)) {
+                            ++count;
+                            s *= 10;
+                        }
+                        // If s is now 1.25, we want to show some more digits,
+                        // but don't want to get silly with a step like 1./7;
+                        // so only include a few extra digits.
+                        const int stop = count + 3;
+                        const auto fractional = [](double v) {
+                            double whole = 0.0;
+                            std::modf(v + 0.5, &whole);
+                            return qAbs(v - whole);
+                        };
+                        s = fractional(s);
+                        while (count < stop && !qFuzzyIsNull(s)) {
+                            ++count;
+                            s = fractional(s * 10);
+                        }
+                        return count;
+                    }(step);
+                    valueStr = qFuzzyIsNull(val / step) ? u"0"_qs
+                                                        : QString::number(val, 'f', precision);
+                }
+            } else {
+                valueStr = valueVar.toString();
+            }
         }
         return valueStr;
     }