gui: Improve support for fractional scaling in command widget on Wind…

…ows.

For long lines, the incorrect character might be repainted when moving the
cursor.  Use fractional character widths to derive the index of the character
in a line at the current cursor position in pixels.

* libgui/qterminal/libqterminal/win32/QWinTerminalImpl.cpp
(QConsolePrivate::m_charSize): Use floating point type QSizeF.
(QConsolePrivate::updateConsoleSize): Determine (approximate) average character
width of used font in floating point precision.
(QConsolePrivate::posToCell, QConsolePrivate::cursorRect,
QConsolePrivate::boundingRect, QWinTerminalImpl::viewPaintEvent): Adapt to
using average character width in floating point precision.

libgui/qterminal/libqterminal/win32/QWinTerminalImpl.cpp

@@ -21,6 +21,7 @@ see <https://www.gnu.org/licenses/>.
 */
 
 #include <algorithm>
+#include <cmath>
 #include <csignal>
 #include <cstdio>
 #include <cstdarg>
@@ -35,6 +36,7 @@ see <https://www.gnu.org/licenses/>.
 #include <io.h>
 #include <versionhelpers.h>
 
+#include <QtDebug>
 #include <QApplication>
 #include <QClipboard>
 #include <QColor>
@@ -44,7 +46,8 @@ see <https://www.gnu.org/licenses/>.
 #include <QPainter>
 #include <QResizeEvent>
 #include <QScrollBar>
-#include <QtDebug>
+#include <QSize>
+#include <QSizeF>
 #include <QThread>
 #include <QTimer>
 #include <QToolTip>
@@ -237,7 +240,7 @@ class QConsolePrivate
   bool m_inWheelEvent;
   QString m_title;
 
-  QSize m_charSize;
+  QSizeF m_charSize;
   QSize m_bufferSize;
   QRect m_consoleRect;
   bool m_auto_scroll;
@@ -494,10 +497,12 @@ void QConsolePrivate::setupStandardIO (DWORD stdHandleId, int targetFd,
 
 QPoint QConsolePrivate::posToCell (const QPoint& p)
 {
-  // FIXME: This might become inaccurate for very long lines if the actual
-  //        character width is fractional.
-  return QPoint (m_consoleRect.left () + p.x () / m_charSize.width (),
-                 m_consoleRect.top () + p.y () / m_charSize.height ());
+  return QPoint (m_consoleRect.left ()
+                 + std::round (static_cast<qreal> (p.x ())
+                               / m_charSize.width ()),
+                 m_consoleRect.top ()
+                 + std::round (static_cast<qreal> (p.y ())
+                               / m_charSize.height ()));
 }
 
 QString QConsolePrivate::getSelection (void)
@@ -1019,11 +1024,22 @@ void QConsolePrivate::updateConsoleSize (bool sync, bool allow_smaller_width)
   QFontMetrics fm = m_consoleView->fontMetrics ();
   QSize winSize = m_consoleView->size ();
 
-  m_charSize.rwidth () = fm.averageCharWidth ();
+  // QFontMetrics::averageCharWidth returns the average character width of the
+  // used font rounded(?) to the nearest integer.  However, the actual
+  // character width might be fractional on screens with non-scalar DPI
+  // scaling.  Take a large sample and divide by the number of characters in
+  // the sample to get a more accurate (fractional) average character width of
+  // the used font.
+  QString sample ('m');
+  sample = sample.repeated (160);  // Is 160 a large enough sample?
+  m_charSize.rwidth ()
+    = static_cast<qreal> (fm.horizontalAdvance (sample)) / 160.;
   m_charSize.rheight () = fm.lineSpacing ();
 
-  m_consoleRect.setWidth (winSize.width () / m_charSize.width ());
-  m_consoleRect.setHeight (winSize.height () / m_charSize.height ());
+  m_consoleRect.setWidth (std::floor (static_cast<qreal> (winSize.width ())
+                                      / m_charSize.width ()));
+  m_consoleRect.setHeight (std::floor (static_cast<qreal> (winSize.height ())
+                                       / m_charSize.height ()));
 
   // Don't shrink the size of the buffer.  That way wide lines won't be
   // truncated and will reappear if the window is enlarged again later.
@@ -1466,13 +1482,13 @@ QConsolePrivate::cursorRect (void)
 
   // Integer precision is good enough for the line height and for the
   // dimensions of the marker.
-  int cw = m_charSize.width ();
-  int ch = m_charSize.height ();
+  qreal cw = m_charSize.width ();
+  qreal ch = m_charSize.height ();
 
   // Make sure the cursor starts *right* of the previous character.
   return QRect (fm.horizontalAdvance (sample),
                 (m_cursorPos.y () - m_consoleRect.y ()) * ch,
-                cw, ch);
+                std::round (cw), ch);
 }
 
 QRect
@@ -1487,12 +1503,12 @@ QConsolePrivate::boundingRect (void)
 
   // Integer precision is good enough for the line height and for the
   // dimensions of the marker.
-  int cw = m_charSize.width ();
-  int ch = m_charSize.height ();
+  qreal cw = m_charSize.width ();
+  qreal ch = m_charSize.height ();
 
   return QRect (fm.horizontalAdvance (sample)-1,
                 (m_cursorPos.y () - m_consoleRect.y ()) * ch,
-                cw+2, ch);
+                std::round (cw+2), ch);
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -1593,16 +1609,18 @@ void QWinTerminalImpl::viewPaintEvent (QConsoleView *w, QPaintEvent *event)
 {
   QPainter p (w);
 
-  int cw = d->m_charSize.width ();
-  int ch = d->m_charSize.height ();
+  qreal cw = d->m_charSize.width ();
+  qreal ch = d->m_charSize.height ();
 
   QRect updateRect = event->rect ();
   p.fillRect (updateRect, QBrush (d->backgroundColor ()));
 
-  int cx1 = updateRect.left () / cw;
+  int cx1 = std::round (static_cast<qreal> (updateRect.left ()) / cw);
   int cy1 = updateRect.top () / ch;
-  int cx2 = qMin (d->m_consoleRect.width () - 1, updateRect.right () / cw);
-  int cy2 = qMin (d->m_consoleRect.height () - 1, updateRect.bottom () / ch);
+  int cx2 = std::round (static_cast<qreal> (updateRect.right ()) / cw);
+  cx2 = qMin (d->m_consoleRect.width () - 1, cx2);
+  int cy2 = std::round (static_cast<qreal> (updateRect.bottom ()) / ch);
+  cy2 = qMin (d->m_consoleRect.height () - 1, cy2);
 
   if (cx1 > d->m_consoleRect.width () - 1
       || cy1 > d->m_consoleRect.height () - 1)