PANEL_PREVIEW_3D_MODEL_BASE: fix opacity slider.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19990

3d-viewer/dialogs/panel_preview_3d_model_base.cpp

@@ -158,7 +158,7 @@ PANEL_PREVIEW_3D_MODEL_BASE::PANEL_PREVIEW_3D_MODEL_BASE( wxWindow* parent, wxWi
 	sbSizer4 = new wxStaticBoxSizer( new wxStaticBox( this, wxID_ANY, _("Opacity") ), wxVERTICAL );
 
 	m_opacity = new wxSlider( sbSizer4->GetStaticBox(), wxID_ANY, 50, 0, 100, wxDefaultPosition, wxDefaultSize, wxSL_HORIZONTAL );
-	sbSizer4->Add( m_opacity, 0, wxRIGHT|wxLEFT, 2 );
+	sbSizer4->Add( m_opacity, 0, wxRIGHT|wxLEFT|wxEXPAND, 2 );
 
 
 	bSizerLeft->Add( sbSizer4, 0, wxBOTTOM|wxEXPAND|wxLEFT|wxRIGHT|wxTOP, 5 );

3d-viewer/dialogs/panel_preview_3d_model_base.fbp

@@ -1889,7 +1889,7 @@
                 <property name="permission">none</property>
                 <object class="sizeritem" expanded="true">
                   <property name="border">2</property>
-                  <property name="flag">wxRIGHT|wxLEFT</property>
+                  <property name="flag">wxRIGHT|wxLEFT|wxEXPAND</property>
                   <property name="proportion">0</property>
                   <object class="wxSlider" expanded="true">
                     <property name="BottomDockable">1</property>