gui: only show the toolbox on enter

This is using opacity of 0% to hide them - this way the space stays allocated
and showing it doesn't rearrange things. Using visiblity here causes
everything to shift around on reveal.

tuhigui/data/ui/Drawing.ui

@@ -22,7 +22,13 @@
     <property name="can_focus">False</property>
     <property name="icon_name">object-rotate-right-symbolic</property>
   </object>
-  <template class="Drawing" parent="GtkBox">
+  <template class="Drawing" parent="GtkEventBox">
+    <property name="visible">True</property>
+    <property name="can_focus">False</property>
+    <signal name="enter-notify-event" handler="_on_enter" swapped="no"/>
+    <signal name="leave-notify-event" handler="_on_leave" swapped="no"/>
+    <child>
+      <object class="GtkBox" id="box_drawing">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
         <property name="margin_left">20</property>
@@ -30,6 +36,7 @@
         <property name="orientation">vertical</property>
         <child>
           <object class="GtkBox" id="box_toolbar">
+            <property name="height_request">20</property>
             <property name="visible">True</property>
             <property name="can_focus">False</property>
             <property name="halign">center</property>
@@ -138,5 +145,7 @@
         <style>
           <class name="drawing"/>
         </style>
+      </object>
+    </child>
   </template>
 </interface>

tuhigui/drawing.py

@@ -12,7 +12,7 @@
 #
 
 from gettext import gettext as _
-from gi.repository import GObject, Gtk, GdkPixbuf
+from gi.repository import GObject, Gtk, GdkPixbuf, Gdk
 
 from .config import Config
 from .svg import JsonSvg
@@ -24,7 +24,7 @@ gi.require_version("Gtk", "3.0")
 
 
 @Gtk.Template(resource_path='/org/freedesktop/Tuhi/ui/Drawing.ui')
-class Drawing(Gtk.Box):
+class Drawing(Gtk.EventBox):
     __gtype_name__ = "Drawing"
 
     box_toolbar = Gtk.Template.Child()
@@ -42,6 +42,7 @@ class Drawing(Gtk.Box):
         self.refresh()  # sets self.svg
 
         self.timestamp = self.svg.timestamp
+        self.box_toolbar.set_opacity(0)
 
     def _on_orientation_changed(self, config, pspec):
         self.orientation = config.orientation
@@ -123,3 +124,13 @@ class Drawing(Gtk.Box):
         o = orientations[orientations.index(self.orientation) + advance]
         self.orientation = o
         self.refresh()
+
+    @Gtk.Template.Callback('_on_enter')
+    def _on_enter(self, *args):
+        self.box_toolbar.set_opacity(100)
+
+    @Gtk.Template.Callback('_on_leave')
+    def _on_leave(self, drawing, event):
+        if event.detail == Gdk.NotifyType.INFERIOR:
+            return
+        self.box_toolbar.set_opacity(0)