Fix #3011: Support left- and right-alignment for multi-line messages and notes

Previously, messages and notes that had multiple lines (via `<br>`-tags) were only displayed correctly
when using the default `center` value for the `messageAlign` and `noteAlign` configuration options.
Using `left` or `right` for the alignment options caused the text to collapse and become illegible,
as outlined in issue #3011.

This comes as a side-effect from how the internal `valign` text-rendering option was configured for
messages and notes:

```js
// Example from `sequenceRenderer.js: drawMessage()`
textObj.anchor = conf.messageAlign;
textObj.valign = conf.messageAlign;
```

Both the `anchor` option (which controls left-right alignment) and the `valign` option (which controls
vertical top-down placement) were set to the same value, the user-provided `messageAlign` config option.

While `left` and `right` are valid values for the `anchor` option, they were effectively no-ops for the
`valign` option, which only supports `top`, `start`, `middle`, `center`, `bottom`, and `end`.

To fix the issue, the `valign` property is now always set to `center` for notes and messages.
Similarly, the `dominantBaseline` option of texts is now always set to `middle`, rather than setting it to
either `text-{before,after}-edge`, which caused left-aligned multi-line text to always be "top-left" aligned
inside a note (or "bottom-right" when attempting to right-align).

Now, texts in messages and notes are always vertically centered and alignment options correctly apply for
both single and multi-line content.

cypress/e2e/rendering/sequencediagram.spec.js

@@ -126,6 +126,17 @@ context('Sequence diagram', () => {
         { sequence: { noteAlign: 'left' } }
       );
     });
+    it('should render multi-line notes aligned to the left when configured', () => {
+      imgSnapshotTest(
+        `
+        sequenceDiagram
+        Alice->>Bob: I'm short
+        note left of Alice: I am left aligned<br>but also<br>multiline
+        Bob->>Alice: Short as well
+      `,
+        { sequence: { noteAlign: 'left' } }
+      );
+    });
     it('should render notes aligned to the right when configured', () => {
       imgSnapshotTest(
         `
@@ -137,6 +148,37 @@ context('Sequence diagram', () => {
         { sequence: { noteAlign: 'right' } }
       );
     });
+    it('should render multi-line notes aligned to the right when configured', () => {
+      imgSnapshotTest(
+        `
+        sequenceDiagram
+        Alice->>Bob: I'm short
+        note left of Alice: I am right aligned<br>but also<br>multiline
+        Bob->>Alice: Short as well
+      `,
+        { sequence: { noteAlign: 'right' } }
+      );
+    });
+    it('should render multi-line messages aligned to the left when configured', () => {
+      imgSnapshotTest(
+        `
+        sequenceDiagram
+        Alice->>Bob: I'm short<br>but also<br>multiline
+        Bob->>Alice: Short as well<br>and also<br>multiline
+      `,
+        { sequence: { messageAlign: 'left' } }
+      );
+    });
+    it('should render multi-line messages aligned to the right when configured', () => {
+      imgSnapshotTest(
+        `
+        sequenceDiagram
+        Alice->>Bob: I'm short<br>but also<br>multiline
+        Bob->>Alice: Short as well<br>and also<br>multiline
+      `,
+        { sequence: { messageAlign: 'right' } }
+      );
+    });
   });
   context('auth width scaling', () => {
     it('should render long actor descriptions', () => {

src/diagrams/sequence/sequenceRenderer.js

@@ -230,7 +230,7 @@ const drawNote = function (elem, noteModel) {
   textObj.fontWeight = conf.noteFontWeight;
   textObj.anchor = conf.noteAlign;
   textObj.textMargin = conf.noteMargin;
-  textObj.valign = conf.noteAlign;
+  textObj.valign = 'center';
 
   let textElem = drawText(g, textObj);
 
@@ -342,7 +342,7 @@ const drawMessage = function (diagram, msgModel, lineStarty) {
   textObj.fontSize = conf.messageFontSize;
   textObj.fontWeight = conf.messageFontWeight;
   textObj.anchor = conf.messageAlign;
-  textObj.valign = conf.messageAlign;
+  textObj.valign = 'center';
   textObj.textMargin = conf.wrapPadding;
   textObj.tspan = false;
 

src/diagrams/sequence/svgDraw.js

@@ -193,7 +193,7 @@ export const drawText = function (elem, textData) {
       case 'start':
         textData.x = Math.round(textData.x + textData.textMargin);
         textData.anchor = 'start';
-        textData.dominantBaseline = 'text-after-edge';
+        textData.dominantBaseline = 'middle';
         textData.alignmentBaseline = 'middle';
         break;
       case 'middle':
@@ -207,7 +207,7 @@ export const drawText = function (elem, textData) {
       case 'end':
         textData.x = Math.round(textData.x + textData.width - textData.textMargin);
         textData.anchor = 'end';
-        textData.dominantBaseline = 'text-before-edge';
+        textData.dominantBaseline = 'middle';
         textData.alignmentBaseline = 'middle';
         break;
     }