Fixed PR to fix popup position if popup is triggered from within an offsetParent element (#7013)

* Fix popup location for tables

This commit introduces the `popupAbsCoords` option to the $button widget
and implements an absolut coordinate format.

Coordinates for popups are stored in the format `(x,y,w,h)`. These
coordinates are relative to the offset parent of the element that
defines the popup.

This commits adds a second format `@(x,y,w,h)`. Coordinates specified in
this format a relative to the pages root element.

The `popupAbsCoords` option of the $button widget enables the use of
this coordinates.

* Unify the declaration of the RegEx for parsing the popup-position

The regular expression was declared in three locations with the same
content. This commit supplies a new function `parseCoordinates` in
`popup.js`. This function returns the parsed coordinates and understands
the classic/absolute coordinates.

This function is used in `reveal.js` and `action-popup.js` to parse the
coordinates.

* Add documentation for coordinate systems

* Consolidate creating coordinate strings

The Popup object now contains a `buildCoordinates` method that can be
used to build coordinate strings. It takes an "enum" for the coordinate-
system to use. This makes everything easily extensible and prevents the
use of magic values.

* Add tests for `parseCoordinates` and `buildCoordinates`

* Add `tv-popup-abs-coords` to `collectDOMVariables`

This will make the absolute coordinates available for the
`DraggableWidget` and the `EventCatcherWidget`.

* Add documentation for the `tv-popup-abs-coords`

... to the `DraggableWidget` and the `EventCatcherWidget`.

* Fix crash when generating a static version of the TW

The Popup class is not initialized in `startup.js` if `$tw.browser` is
not true. After having consolidated the facilities for parsing
coordinate strings into `popup.js` this breaks because the static build
needs to parse coordinate stings even if no Popup module is initialized.
This commit solves this problem by making `readPopupState`,
`parseCoordinates` and `buildCoordinates` static methods of `popup.js`.
It also adds a comment to these functions to show that these can be called
safely even if the Popup-Class is not initialized.

core/modules/widgets/button.js

@@ -14,6 +14,8 @@ Button widget
 
 var Widget = require("$:/core/modules/widgets/widget.js").widget;
 
+var Popup = require("$:/core/modules/utils/dom/popup.js");
+
 var ButtonWidget = function(parseTreeNode,options) {
 	this.initialise(parseTreeNode,options);
 };
@@ -147,7 +149,7 @@ ButtonWidget.prototype.isSelected = function() {
 
 ButtonWidget.prototype.isPoppedUp = function() {
 	var tiddler = this.popupTitle ? this.wiki.getTiddler(this.popupTitle) : this.wiki.getTiddler(this.popup);
-	var result = tiddler && tiddler.fields.text ? $tw.popup.readPopupState(tiddler.fields.text) : false;
+	var result = tiddler && tiddler.fields.text ? Popup.readPopupState(tiddler.fields.text) : false;
 	return result;
 };
 
@@ -173,6 +175,7 @@ ButtonWidget.prototype.triggerPopup = function(event) {
 	if(this.popupTitle) {
 		$tw.popup.triggerPopup({
 			domNode: this.domNodes[0],
+			absolute: (this.popupAbsCoords === "yes"),
 			title: this.popupTitle,
 			wiki: this.wiki,
 			noStateReference: true
@@ -180,6 +183,7 @@ ButtonWidget.prototype.triggerPopup = function(event) {
 	} else {
 		$tw.popup.triggerPopup({
 			domNode: this.domNodes[0],
+			absolute: (this.popupAbsCoords === "yes"),
 			title: this.popup,
 			wiki: this.wiki
 		});
@@ -223,6 +227,7 @@ ButtonWidget.prototype.execute = function() {
 	this.setField = this.getAttribute("setField");
 	this.setIndex = this.getAttribute("setIndex");
 	this.popupTitle = this.getAttribute("popupTitle");
+	this.popupAbsCoords = this.getAttribute("popupAbsCoords", "no");
 	this.tabIndex = this.getAttribute("tabindex");
 	this.isDisabled = this.getAttribute("disabled","no");
 	// Make child widgets
@@ -252,7 +257,7 @@ Selectively refreshes the widget if needed. Returns true if the widget or any of
 */
 ButtonWidget.prototype.refresh = function(changedTiddlers) {
 	var changedAttributes = this.computeAttributes();
-	if(changedAttributes.actions || changedAttributes.to || changedAttributes.message || changedAttributes.param || changedAttributes.set || changedAttributes.setTo || changedAttributes.popup || changedAttributes.hover || changedAttributes.selectedClass || changedAttributes.style || changedAttributes.dragFilter || changedAttributes.dragTiddler || (this.set && changedTiddlers[this.set]) || (this.popup && changedTiddlers[this.popup]) || (this.popupTitle && changedTiddlers[this.popupTitle]) || changedAttributes.setTitle || changedAttributes.setField || changedAttributes.setIndex || changedAttributes.popupTitle || changedAttributes.disabled || changedAttributes["default"]) {
+	if(changedAttributes.actions || changedAttributes.to || changedAttributes.message || changedAttributes.param || changedAttributes.set || changedAttributes.setTo || changedAttributes.popup || changedAttributes.hover || changedAttributes.selectedClass || changedAttributes.style || changedAttributes.dragFilter || changedAttributes.dragTiddler || (this.set && changedTiddlers[this.set]) || (this.popup && changedTiddlers[this.popup]) || (this.popupTitle && changedTiddlers[this.popupTitle]) || changedAttributes.popupAbsCoords || changedAttributes.setTitle || changedAttributes.setField || changedAttributes.setIndex || changedAttributes.popupTitle || changedAttributes.disabled || changedAttributes["default"]) {
 		this.refreshSelf();
 		return true;
 	} else if(changedAttributes["class"]) {