Add clampToContainer property for containing-block clamping

Expose a reflected clampToContainer boolean that, when set, makes fit()
clamp the popup against its nearest containing-block rect instead of the
viewport. Required for popups under an ancestor with transform/filter/
contain:layout|paint, where position: fixed resolves against that ancestor.

Bumps @tp/helpers dependency to ^2.13.0 for the matching _posFixed support.

tp-popup.js

@@ -89,7 +89,12 @@ class TpPopup extends EventHelpers(Position(DomQuery(LitElement))) {
       alwaysToggle: { type: Boolean, reflect: true },
       halign: { type: String },
       valign: { type: String },
-      scrollTarget: { type: Object }
+      scrollTarget: { type: Object },
+      // When true, clamp the popup against its containing-block rect instead
+      // of the viewport. Enable for popups under an ancestor with transform /
+      // filter / contain:layout|paint, where position: fixed resolves against
+      // that ancestor and the viewport is the wrong clamp edge.
+      clampToContainer: { type: Boolean, reflect: true }
     };
   }
 
@@ -100,6 +105,7 @@ class TpPopup extends EventHelpers(Position(DomQuery(LitElement))) {
     this.halign = 'middle';
     this.valign = 'bottom';
     this.scrollTarget = document;
+    this.clampToContainer = false;
     this.fit = this.fit.bind(this);
     this._onDocClickHandler = () => this.close();
   }
@@ -165,7 +171,8 @@ class TpPopup extends EventHelpers(Position(DomQuery(LitElement))) {
     this._posFixed(this.shadowRoot.querySelector('.toggle'), this.$.content, {
       valign: this.valign || 'bottom',
       halign: this.halign || 'middle',
-      spacing: 5
+      spacing: 5,
+      clampToContainer: this.clampToContainer
     });
   }