slider: Request a size

Sliders (and level bars) currently don't request any size, so it's
up to the caller (or stylesheet) to explicitly set a size big
enough to fit the bar/handle.

That's rather unexpected, we should request the size we need ourselves.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7133

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2994>

js/ui/barLevel.js

@@ -259,6 +259,26 @@ export const BarLevel = GObject.registerClass({
         cr.$dispose();
     }
 
+    vfunc_get_preferred_height(_forWidth) {
+        const themeNode = this.get_theme_node();
+        const height = this._getPreferredHeight();
+        return themeNode.adjust_preferred_height(height, height);
+    }
+
+    vfunc_get_preferred_width(_forHeight) {
+        const themeNode = this.get_theme_node();
+        const width = this._getPreferredWidth();
+        return themeNode.adjust_preferred_width(width, width);
+    }
+
+    _getPreferredHeight() {
+        return this._barLevelHeight + this._barLevelBorderWidth;
+    }
+
+    _getPreferredWidth() {
+        return this._overdriveSeparatorWidth + this._barLevelBorderWidth;
+    }
+
     _getCurrentValue() {
         return this._value;
     }