preserve Arabic medials when line-wrapping

can't believe I never caught this! it's a bit rare since you have to
break a word rather than splitting on a space.

Author: chearon

src/layout-text.ts

@@ -2015,11 +2015,18 @@ function shapePart(
   face: LoadedFontFace,
   attrs: ShapingAttrs
 ) {
-  if (face.spaceMayParticipateInShaping(attrs.script)) {
-    return shapePartWithoutWordCache(block, offset, length, face.hbfont, attrs);
-  } else {
-    return shapePartWithWordCache(block, offset, length, face.hbfont, attrs);
+  if (!face.spaceMayParticipateInShaping(attrs.script)) {
+    const t = block.text;
+    const end = offset + length;
+    if (
+      (offset === 0 || t[offset] === ' ' || t[offset - 1] === ' ') &&
+      (end === t.length || t[end] === ' ' || t[end - 1] === ' ')
+    ) {
+      return shapePartWithWordCache(block, offset, length, face.hbfont, attrs);
+    }
   }
+
+  return shapePartWithoutWordCache(block, offset, length, face.hbfont, attrs);
 }
 
 export function createIfcShapedItems(

test/text.spec.js

@@ -1084,15 +1084,17 @@ describe('Lines', function () {
     expect(inline.lineboxes[1].head.value.offset).to.equal(12);
   });
 
-  it('adds a soft hyphen to RTL text if one fits after a &shy', function () {
+  it('adds a soft hyphen to Arabic and keeps medial form', function () {
     this.reflow(`
       <div id="t" style="direction: rtl; font: 24px Cairo; width: 51px;">
         دامي&shy;دى
       </div>
     `);
     /** @type import('../src/layout-flow.ts').BlockContainerOfInlines */
     const inline = this.get('div');
-    expect(inline.lineboxes[0].head.value.glyphs[0 * G_SZ + G_ID]).to.equal(672);
+    expect(inline.lineboxes[0].head.value.glyphs[0 * G_SZ + G_ID]).to.equal(672); // hyphen
+    expect(inline.lineboxes[0].head.value.glyphs[1 * G_SZ + G_ID]).to.equal(697); // shy
+    expect(inline.lineboxes[0].head.value.glyphs[2 * G_SZ + G_ID]).to.equal(441); // yeh, medial
     expect(inline.lineboxes[1].head.value.offset).to.equal(6);
   });