Fix very tall Plot dialog on GTK.

Some scrollable controls need wxFIXED_MINSIZE, on some platforms, apparently.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16926

pcbnew/dialogs/dialog_plot.cpp

@@ -160,7 +160,7 @@ DIALOG_PLOT::DIALOG_PLOT( PCB_EDIT_FRAME* aParent ) :
         m_plotAllLayersList->SetClientObject( list_idx, new PCB_LAYER_ID_CLIENT_DATA( layer_id ) );
     }
 
-	sbSizer->Add( m_plotAllLayersList, 1, wxALL | wxEXPAND, 3 );
+	sbSizer->Add( m_plotAllLayersList, 1, wxALL | wxEXPAND | wxFIXED_MINSIZE, 3 );
 
 	wxBoxSizer* bButtonSizer;
 	bButtonSizer = new wxBoxSizer( wxHORIZONTAL );

pcbnew/dialogs/dialog_plot_base.cpp

@@ -57,7 +57,7 @@ DIALOG_PLOT_BASE::DIALOG_PLOT_BASE( wxWindow* parent, wxWindowID id, const wxStr
 	m_layerCheckListBox = new wxCheckListBox( m_LayersSizer->GetStaticBox(), wxID_ANY, wxDefaultPosition, wxDefaultSize, m_layerCheckListBoxChoices, 0 );
 	m_layerCheckListBox->SetMinSize( wxSize( 150,-1 ) );
 
-	m_LayersSizer->Add( m_layerCheckListBox, 1, wxALL|wxEXPAND, 3 );
+	m_LayersSizer->Add( m_layerCheckListBox, 1, wxALL|wxEXPAND|wxFIXED_MINSIZE, 3 );
 
 
 	bmiddleSizer->Add( m_LayersSizer, 1, wxALL|wxEXPAND, 5 );

pcbnew/dialogs/dialog_plot_base.fbp

@@ -431,7 +431,7 @@
                                 <property name="permission">protected</property>
                                 <object class="sizeritem" expanded="1">
                                     <property name="border">3</property>
-                                    <property name="flag">wxALL|wxEXPAND</property>
+                                    <property name="flag">wxALL|wxEXPAND|wxFIXED_MINSIZE</property>
                                     <property name="proportion">1</property>
                                     <object class="wxCheckListBox" expanded="1">
                                         <property name="BottomDockable">1</property>