panels: implement a small margin on all panels to ensure the focus border is always visible

app/widget/curvewidget/curvewidget.cpp

@@ -39,6 +39,7 @@ CurveWidget::CurveWidget(QWidget *parent) :
   bridge_(nullptr)
 {
   QVBoxLayout* layout = new QVBoxLayout(this);
+  layout->setMargin(0);
 
   QHBoxLayout* top_controls = new QHBoxLayout();
 

app/widget/panel/panel.cpp

@@ -22,6 +22,7 @@
 
 #include <QCloseEvent>
 #include <QDebug>
+#include <QHBoxLayout>
 #include <QPainter>
 #include <QStyle>
 #include <QStyleOption>
@@ -144,4 +145,13 @@ void PanelWidget::Retranslate()
 {
 }
 
+void PanelWidget::SetWidgetWithPadding(QWidget *widget)
+{
+  QWidget* wrapper = new QWidget();
+  QHBoxLayout* layout = new QHBoxLayout(wrapper);
+  layout->setMargin(layout->margin() / 2);
+  layout->addWidget(widget);
+  setWidget(wrapper);
+}
+
 OLIVE_NAMESPACE_EXIT

app/widget/panel/panel.h

@@ -181,6 +181,8 @@ protected:
 
   virtual void Retranslate();
 
+  void SetWidgetWithPadding(QWidget* widget);
+
 protected slots:
   /**
    * @brief Set panel's title

app/widget/viewer/pixelsamplerwidget.cpp

@@ -60,6 +60,7 @@ ManagedPixelSamplerWidget::ManagedPixelSamplerWidget(QWidget *parent) :
   QWidget(parent)
 {
   QVBoxLayout* layout = new QVBoxLayout(this);
+  layout->setMargin(0);
 
   display_view_ = new PixelSamplerWidget();
   display_view_->setTitle(tr("Display"));