qt: Remove "Pay only required fee" checkbox

The custom fee input box now has a minimum value equal to the minimum
required fee. Before a value below the minimum fee could be entered
which was confusing since the minimum fee would still be paid even
though a lower amount was entered.

src/qt/forms/sendcoinsdialog.ui

@@ -878,28 +878,15 @@ Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis p
                <item>
                 <layout class="QHBoxLayout" name="horizontalLayoutFee8">
                  <item>
-                  <widget class="QCheckBox" name="checkBoxMinimumFee">
+                  <widget class="QLabel" name="labelCustomFeeWarning">
-                   <property name="toolTip">
-                    <string>Paying only the minimum fee is just fine as long as there is less transaction volume than space in the blocks. But be aware that this can end up in a never confirming transaction once there is more demand for bitcoin transactions than the network can process.</string>
-                   </property>
-                   <property name="text">
-                    <string/>
-                   </property>
-                  </widget>
-                 </item>
-                 <item>
-                  <widget class="QLabel" name="labelMinFeeWarning">
                    <property name="enabled">
                     <bool>true</bool>
                    </property>
                    <property name="toolTip">
-                    <string>Paying only the minimum fee is just fine as long as there is less transaction volume than space in the blocks. But be aware that this can end up in a never confirming transaction once there is more demand for bitcoin transactions than the network can process.</string>
+                    <string>When there is less transaction volume than space in the blocks, miners as well as relaying nodes may enforce a minimum fee. Paying only this minimum fee is just fine, but be aware that this can result in a never confirming transaction once there is more demand for bitcoin transactions than the network can process.</string>
                    </property>
                    <property name="text">
-                    <string>(read the tooltip)</string>
+                    <string>A too low fee might result in a never confirming transaction (read the tooltip)</string>
-                   </property>
-                   <property name="margin">
-                    <number>5</number>
                    </property>
                   </widget>
                  </item>
@@ -992,9 +979,6 @@ Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis p
                    <property name="text">
                     <string/>
                    </property>
-                   <property name="margin">
-                    <number>2</number>
-                   </property>
                   </widget>
                  </item>
                  <item>
@@ -1009,9 +993,6 @@ Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis p
                    <property name="text">
                     <string>(Smart fee not initialized yet. This usually takes a few blocks...)</string>
                    </property>
-                   <property name="margin">
-                    <number>2</number>
-                   </property>
                   </widget>
                  </item>
                  <item>
@@ -1038,24 +1019,8 @@ Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis p
                      <property name="text">
                       <string>Confirmation time target:</string>
                      </property>
-                     <property name="margin">
-                      <number>2</number>
-                     </property>
                     </widget>
                    </item>
-                   <item>
-                    <spacer name="verticalSpacer_3">
-                     <property name="orientation">
-                      <enum>Qt::Vertical</enum>
-                     </property>
-                     <property name="sizeHint" stdset="0">
-                      <size>
-                       <width>1</width>
-                       <height>1</height>
-                      </size>
-                     </property>
-                    </spacer>
-                   </item>
                   </layout>
                  </item>
                  <item>

src/qt/sendcoinsdialog.cpp

@@ -119,13 +119,11 @@ SendCoinsDialog::SendCoinsDialog(const PlatformStyle *_platformStyle, QWidget *p
         settings.setValue("nSmartFeeSliderPosition", 0);
     if (!settings.contains("nTransactionFee"))
         settings.setValue("nTransactionFee", (qint64)DEFAULT_PAY_TX_FEE);
-    if (!settings.contains("fPayOnlyMinFee"))
-        settings.setValue("fPayOnlyMinFee", false);
     ui->groupFee->setId(ui->radioSmartFee, 0);
     ui->groupFee->setId(ui->radioCustomFee, 1);
     ui->groupFee->button((int)std::max(0, std::min(1, settings.value("nFeeRadio").toInt())))->setChecked(true);
+    ui->customFee->SetAllowEmpty(false);
     ui->customFee->setValue(settings.value("nTransactionFee").toLongLong());
-    ui->checkBoxMinimumFee->setChecked(settings.value("fPayOnlyMinFee").toBool());
     minimizeFeeSection(settings.value("fFeeSectionMinimized").toBool());
 }
 
@@ -174,14 +172,15 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         connect(ui->groupFee, static_cast<void (QButtonGroup::*)(int)>(&QButtonGroup::buttonClicked), this, &SendCoinsDialog::updateFeeSectionControls);
         connect(ui->groupFee, static_cast<void (QButtonGroup::*)(int)>(&QButtonGroup::buttonClicked), this, &SendCoinsDialog::coinControlUpdateLabels);
         connect(ui->customFee, &BitcoinAmountField::valueChanged, this, &SendCoinsDialog::coinControlUpdateLabels);
-        connect(ui->checkBoxMinimumFee, &QCheckBox::stateChanged, this, &SendCoinsDialog::setMinimumFee);
-        connect(ui->checkBoxMinimumFee, &QCheckBox::stateChanged, this, &SendCoinsDialog::updateFeeSectionControls);
-        connect(ui->checkBoxMinimumFee, &QCheckBox::stateChanged, this, &SendCoinsDialog::coinControlUpdateLabels);
         connect(ui->optInRBF, &QCheckBox::stateChanged, this, &SendCoinsDialog::updateSmartFeeLabel);
         connect(ui->optInRBF, &QCheckBox::stateChanged, this, &SendCoinsDialog::coinControlUpdateLabels);
-        ui->customFee->setSingleStep(model->wallet().getRequiredFee(1000));
+        CAmount requiredFee = model->wallet().getRequiredFee(1000);
+        ui->customFee->SetMinValue(requiredFee);
+        if (ui->customFee->value() < requiredFee) {
+            ui->customFee->setValue(requiredFee);
+        }
+        ui->customFee->setSingleStep(requiredFee);
         updateFeeSectionControls();
-        updateMinFeeLabel();
         updateSmartFeeLabel();
 
         // set default rbf checkbox state
@@ -210,7 +209,6 @@ SendCoinsDialog::~SendCoinsDialog()
     settings.setValue("nFeeRadio", ui->groupFee->checkedId());
     settings.setValue("nConfTarget", getConfTargetForIndex(ui->confTargetSelector->currentIndex()));
     settings.setValue("nTransactionFee", (qint64)ui->customFee->value());
-    settings.setValue("fPayOnlyMinFee", ui->checkBoxMinimumFee->isChecked());
 
     delete ui;
 }
@@ -542,7 +540,6 @@ void SendCoinsDialog::updateDisplayUnit()
 {
     setBalance(model->wallet().getBalances());
     ui->customFee->setDisplayUnit(model->getOptionsModel()->getDisplayUnit());
-    updateMinFeeLabel();
     updateSmartFeeLabel();
 }
 
@@ -642,11 +639,6 @@ void SendCoinsDialog::useAvailableBalance(SendCoinsEntry* entry)
     }
 }
 
-void SendCoinsDialog::setMinimumFee()
-{
-    ui->customFee->setValue(model->wallet().getRequiredFee(1000));
-}
-
 void SendCoinsDialog::updateFeeSectionControls()
 {
     ui->confTargetSelector      ->setEnabled(ui->radioSmartFee->isChecked());
@@ -654,10 +646,9 @@ void SendCoinsDialog::updateFeeSectionControls()
     ui->labelSmartFee2          ->setEnabled(ui->radioSmartFee->isChecked());
     ui->labelSmartFee3          ->setEnabled(ui->radioSmartFee->isChecked());
     ui->labelFeeEstimation      ->setEnabled(ui->radioSmartFee->isChecked());
-    ui->checkBoxMinimumFee      ->setEnabled(ui->radioCustomFee->isChecked());
+    ui->labelCustomFeeWarning   ->setEnabled(ui->radioCustomFee->isChecked());
-    ui->labelMinFeeWarning      ->setEnabled(ui->radioCustomFee->isChecked());
+    ui->labelCustomPerKilobyte  ->setEnabled(ui->radioCustomFee->isChecked());
-    ui->labelCustomPerKilobyte  ->setEnabled(ui->radioCustomFee->isChecked() && !ui->checkBoxMinimumFee->isChecked());
+    ui->customFee               ->setEnabled(ui->radioCustomFee->isChecked());
-    ui->customFee               ->setEnabled(ui->radioCustomFee->isChecked() && !ui->checkBoxMinimumFee->isChecked());
 }
 
 void SendCoinsDialog::updateFeeMinimizedLabel()
@@ -672,14 +663,6 @@ void SendCoinsDialog::updateFeeMinimizedLabel()
     }
 }
 
-void SendCoinsDialog::updateMinFeeLabel()
-{
-    if (model && model->getOptionsModel())
-        ui->checkBoxMinimumFee->setText(tr("Pay only the required fee of %1").arg(
-            BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), model->wallet().getRequiredFee(1000)) + "/kB")
-        );
-}
-
 void SendCoinsDialog::updateCoinControlState(CCoinControl& ctrl)
 {
     if (ui->radioCustomFee->isChecked()) {

src/qt/sendcoinsdialog.h

@@ -92,9 +92,7 @@ private Q_SLOTS:
     void coinControlClipboardBytes();
     void coinControlClipboardLowOutput();
     void coinControlClipboardChange();
-    void setMinimumFee();
     void updateFeeSectionControls();
-    void updateMinFeeLabel();
     void updateSmartFeeLabel();
 
 Q_SIGNALS: