Add support for rebasing headings at render time

Fixes #3330

core/modules/widgets/element.js

@@ -35,6 +35,14 @@ ElementWidget.prototype.render = function(parent,nextSibling) {
 	if($tw.config.htmlUnsafeElements.indexOf(tag) !== -1) {
 		tag = "safe-" + tag;
 	}
+	// Adjust headings by the current base level
+	var headingLevel = ["h1","h2","h3","h4","h5","h6"].indexOf(tag);
+	if(headingLevel !== -1) {
+		var baseLevel = parseInt(this.getVariable("tv-adjust-heading-level","0"),10) || 0;
+		headingLevel = Math.min(Math.max(headingLevel + 1 + baseLevel,1),6);
+		tag = "h" + headingLevel;
+	}
+	// Create the DOM node
 	var domNode = this.document.createElementNS(this.namespace,tag);
 	this.assignAttributes(domNode,{excludeEventAttributes: true});
 	parent.insertBefore(domNode,nextSibling);

editions/tw5.com/tiddlers/variables/examples/Sample Headings 1-2-3.tid

@@ -0,0 +1,22 @@
+created: 20180609112450207
+modified: 20180609112450207
+title: Sample Headings 1,2,3
+
+! This is a Level 1 Heading
+
+This is a paragraph.
+
+<$vars tv-adjust-heading-level="-2">
+
+{{Sample Headings 3,4,5}}
+
+</$vars>
+
+!! This is a Level 2 heading
+
+<$vars tv-adjust-heading-level="-1">
+
+{{Sample Headings 4,5,6}}
+
+</$vars>
+

editions/tw5.com/tiddlers/variables/examples/Sample Headings 3-4-5.tid

@@ -0,0 +1,9 @@
+created: 20180609112450207
+modified: 20180609112450207
+title: Sample Headings 3,4,5
+
+!!! This is written as a Level 3 heading
+
+!!!! This is written as a Level 4 heading
+
+!!!!! This is written as a Level 5 heading

editions/tw5.com/tiddlers/variables/examples/Sample Headings 4-5-6.tid

@@ -0,0 +1,9 @@
+created: 20180609112450207
+modified: 20180609112450207
+title: Sample Headings 4,5,6
+
+!!!! This is written as a Level 4 heading
+
+!!!!! This is written as a Level 5 heading
+
+!!!!!! This is written as a Level 6 heading

editions/tw5.com/tiddlers/variables/examples/tv-adjust-heading-level.tid

@@ -0,0 +1,25 @@
+created: 20180609112450207
+modified: 20180609112450207
+tags: [[tv-adjust-heading-level Variable]] [[Variable Examples]]
+title: tv-adjust-heading-level Variable (Examples)
+type: text/vnd.tiddlywiki
+
+\define show-tiddler(title)
+!! $title$
+
+<$codeblock code={{$title$}}/>
+\end
+
+! Tiddlers
+
+<<show-tiddler "Sample Headings 1,2,3">>
+
+<<show-tiddler "Sample Headings 3,4,5">>
+
+<<show-tiddler "Sample Headings 4,5,6">>
+
+! Example
+
+<$macrocall $name=".example" n="1"
+eg="""<$transclude tiddler="Sample Headings 1,2,3" mode="block"/>"""/>
+

editions/tw5.com/tiddlers/variables/tv-adjust-heading-level Variable.tid

@@ -0,0 +1,12 @@
+caption: tv-adjust-heading-level
+created: 20180609112450207
+modified: 20180609112450207
+tags: Variables [[Core Variables]] [[Configuration Variables]]
+title: tv-adjust-heading-level Variable
+type: text/vnd.tiddlywiki
+
+The <<.def tv-adjust-heading-level>> [[variable|Variables]] allows the level of [[headings|Headings in WikiText]] to be adjusted. It is interpreted as a positive or negative number (e.g. "2" or "-3") that is added to the heading level for display.
+
+Note that if the resulting heading level is less than 1 then level 1 is used instead. This means that specfiying a large negative adjustment factor will reset all headings to display as level 1. Similarly, heading levels larger than 6 are clamped to the maximum HTML heading level of 6.
+
+<<.variable-examples "tv-adjust-heading-level">>