qml: ensure that the slider neutral position is in the middle of the screen

electrum/gui/qml/components/SwapDialog.qml

@@ -159,8 +159,9 @@ ElDialog {
             id: swapslider
             Layout.topMargin: constants.paddingLarge
             Layout.bottomMargin: constants.paddingLarge
-            Layout.leftMargin: constants.paddingXXLarge
-            Layout.rightMargin: constants.paddingXXLarge
+            Layout.leftMargin: constants.paddingXXLarge + (parent.width - 2 * constants.paddingXXLarge) * swaphelper.leftVoid
+            Layout.rightMargin: constants.paddingXXLarge + (parent.width - 2 * constants.paddingXXLarge) * swaphelper.rightVoid
+
             Layout.fillWidth: true
 
             from: swaphelper.rangeMin

electrum/gui/qml/qeswaphelper.py

@@ -45,6 +45,9 @@ class QESwapHelper(AuthMixin, QObject):
         self._send_amount = 0
         self._receive_amount = 0
 
+        self._leftVoid = 0
+        self._rightVoid = 0
+
     walletChanged = pyqtSignal()
     @pyqtProperty(QEWallet, notify=walletChanged)
     def wallet(self):
@@ -91,6 +94,28 @@ class QESwapHelper(AuthMixin, QObject):
             self._rangeMax = rangeMax
             self.rangeMaxChanged.emit()
 
+    leftVoidChanged = pyqtSignal()
+    @pyqtProperty(float, notify=leftVoidChanged)
+    def leftVoid(self):
+        return self._leftVoid
+
+    @leftVoid.setter
+    def leftVoid(self, leftVoid):
+        if self._leftVoid != leftVoid:
+            self._leftVoid = leftVoid
+            self.leftVoidChanged.emit()
+
+    rightVoidChanged = pyqtSignal()
+    @pyqtProperty(float, notify=rightVoidChanged)
+    def rightVoid(self):
+        return self._rightVoid
+
+    @rightVoid.setter
+    def rightVoid(self, rightVoid):
+        if self._rightVoid != rightVoid:
+            self._rightVoid = rightVoid
+            self.rightVoidChanged.emit()
+
     validChanged = pyqtSignal()
     @pyqtProperty(bool, notify=validChanged)
     def valid(self):
@@ -215,6 +240,16 @@ class QESwapHelper(AuthMixin, QObject):
         self._logger.debug(f'Slider range {-reverse} - {forward}')
         self.rangeMin = -reverse
         self.rangeMax = forward
+        # percentage of void, right or left
+        if reverse < forward:
+            self.leftVoid = 0.5 * (forward - reverse) / forward
+            self.rightVoid = 0
+        elif reverse > forward:
+            self.leftVoid = 0
+            self.rightVoid = - 0.5 * (forward - reverse) / reverse
+        else:
+            self.leftVoid = 0
+            self.rightVoid = 0
 
         self.swap_slider_moved()