PdfMultiPageView: retain position after pinch zoom

If you do the pinch on a trackpad, the mouse cursor doesn't move, and
the same visible part of the page will remain under the cursor after the
page is re-rendered.  Likewise when doing the pinch on a touchscreen,
the centroid (midpoint between the two fingers) is held in place,
subject to the constraints that Flickable.returnToBounds() imposes.
Similar to 29cb44ee

.

Also corrected the horizontal scrolling range according to rotation;
added left and right margins so that the page edges are more visible
when the page is wider than the view, and thus the horizontal scrolling
range feels more accurate while the scrollbars are still shown.

Change-Id: Ia2a15eee5bcd5c9e7f025634f02a5a546e6aab68
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/pdf/quick/qml/PdfMultiPageView.qml

@@ -121,13 +121,14 @@ Item {
     TableView {
         id: tableView
         anchors.fill: parent
+        anchors.leftMargin: 2
         model: root.document === undefined ? 0 : root.document.pageCount
         rowSpacing: 6
-        property real rotationModulus: Math.abs(root.pageRotation % 180)
-        property bool rot90: rotationModulus > 45 && rotationModulus < 135
+        property real rotationNorm: Math.round((360 + (root.pageRotation % 360)) % 360)
+        property bool rot90: rotationNorm == 90 || rotationNorm == 270
         onRot90Changed: forceLayout()
         property size firstPagePointSize: document === undefined ? Qt.size(0, 0) : document.pagePointSize(0)
-        contentWidth: document === undefined ? 0 : document.maxPageWidth * root.renderScale
+        contentWidth: document === undefined ? 0 : (rot90 ? document.maxPageHeight : document.maxPageWidth) * root.renderScale + vscroll.width + 2
         // workaround for missing function (see https://codereview.qt-project.org/c/qt/qtdeclarative/+/248464)
         function itemAtPos(x, y, includeSpacing) {
             // we don't care about x (assume col 0), and assume includeSpacing is true
@@ -139,7 +140,7 @@ Item {
                 if (child.y < y && (!ret || child.y > ret.y))
                     ret = child
             }
-            if (root.debug)
+            if (root.debug && ret !== null)
                 console.log("given y", y, "found", ret, "@", ret.y)
             return ret // the delegate with the largest y that is less than the given y
         }
@@ -164,7 +165,7 @@ Item {
                 width: image.width
                 height: image.height
                 rotation: root.pageRotation
-                anchors.centerIn: parent
+                anchors.centerIn: pinch.active ? undefined : parent
                 property size pagePointSize: document.pagePointSize(index)
                 property real pageScale: image.paintedWidth / pagePointSize.width
                 Image {
@@ -223,19 +224,46 @@ Item {
                     id: pinch
                     minimumScale: 0.1
                     maximumScale: root.renderScale < 4 ? 2 : 1
-                    minimumRotation: 0
-                    maximumRotation: 0
+                    minimumRotation: root.pageRotation
+                    maximumRotation: root.pageRotation
                     enabled: image.sourceSize.width < 5000
                     onActiveChanged:
                         if (active) {
                             paper.z = 10
                         } else {
                             paper.z = 0
+                            var centroidInPoints = Qt.point(pinch.centroid.position.x / root.renderScale,
+                                                            pinch.centroid.position.y / root.renderScale)
+                            var centroidInFlickable = tableView.mapFromItem(paper, pinch.centroid.position.x, pinch.centroid.position.y)
                             var newSourceWidth = image.sourceSize.width * paper.scale
                             var ratio = newSourceWidth / image.sourceSize.width
+                            if (root.debug)
+                                console.log("pinch ended on page", index, "with centroid", pinch.centroid.position, centroidInPoints, "wrt flickable", centroidInFlickable,
+                                            "page at", pageHolder.x.toFixed(2), pageHolder.y.toFixed(2),
+                                            "contentX/Y were", tableView.contentX.toFixed(2), tableView.contentY.toFixed(2))
                             if (ratio > 1.1 || ratio < 0.9) {
+                                var centroidOnPage = Qt.point(centroidInPoints.x * root.renderScale * ratio, centroidInPoints.y * root.renderScale * ratio)
                                 paper.scale = 1
+                                paper.x = 0
+                                paper.y = 0
                                 root.renderScale *= ratio
+                                tableView.forceLayout()
+                                if (tableView.rotationNorm == 0) {
+                                    tableView.contentX = pageHolder.x + tableView.originX + centroidOnPage.x - centroidInFlickable.x
+                                    tableView.contentY = pageHolder.y + tableView.originY + centroidOnPage.y - centroidInFlickable.y
+                                } else if (tableView.rotationNorm == 90) {
+                                    tableView.contentX = pageHolder.x + tableView.originX + image.height - centroidOnPage.y - centroidInFlickable.x
+                                    tableView.contentY = pageHolder.y + tableView.originY + centroidOnPage.x - centroidInFlickable.y
+                                } else if (tableView.rotationNorm == 180) {
+                                    tableView.contentX = pageHolder.x + tableView.originX + image.width - centroidOnPage.x - centroidInFlickable.x
+                                    tableView.contentY = pageHolder.y + tableView.originY + image.height - centroidOnPage.y - centroidInFlickable.y
+                                } else if (tableView.rotationNorm == 270) {
+                                    tableView.contentX = pageHolder.x + tableView.originX + centroidOnPage.y - centroidInFlickable.x
+                                    tableView.contentY = pageHolder.y + tableView.originY + image.width - centroidOnPage.x - centroidInFlickable.y
+                                }
+                                if (root.debug)
+                                    console.log("contentX/Y adjusted to", tableView.contentX.toFixed(2), tableView.contentY.toFixed(2), "y @top", pageHolder.y)
+                                tableView.returnToBounds()
                             }
                         }
                     grabPermissions: PointerHandler.CanTakeOverFromAnything
@@ -298,6 +326,7 @@ Item {
             }
         }
         ScrollBar.vertical: ScrollBar {
+            id: vscroll
             property bool moved: false
             onPositionChanged: moved = true
             onActiveChanged: {