Bug 328044 – [SVG] <textPath href> to a basic shape does not follow later changes to the shape

• Component: SVG
• Status: NEW

[Karl Dubost]

SVG 2 lets a textPath href point at a basic shape as well as a path: 

    "An URL reference to the path element or basic shape element onto which the glyphs will be rendered" 
    (https://w3c.github.io/svgwg/svg2-draft/text.html#TextPathElementHrefAttribute).

WebKit lays the text out on the shape the first time. After that, changing the shape does not move the text. A path target does follow.

Test case: 
a text with <textPath href="#s">, where #s is a rect, circle, ellipse, line or polygon. 
After the first layout, move the shape (for example rect y from 20 to 120), then read getStartPositionOfChar(3).

path (control): y 100 -> 150, follows
rect, circle, ellipse, line, polygon: unchanged

Cause: 
SVGTextPathElement::buildPendingResource() calls addReferencingElement() only when the target is an SVGPathElement (SVGTextPathElement.cpp:180), and SVGPathElement::invalidatePathDependencies() is what tells the textPath. RenderSVGTextPath::layoutPath() already accepts any SVGGeometryElement.

Other engines, same probe: 
* Firefox 160 follows all six. 
* Chrome Canary 158 never puts the text on the shape (position (0,0) before any edit): Blink's LayoutSVGTextPath::LayoutPath() only accepts an SVGPathElement. That is a separate gap, not this bug. I will add more WPT. 

Probe attached.

[Karl Dubost]

Created attachment 481890 [details]
probe-textpath-href-basic-shape-live-update.html

[Karl Dubost]

I also opened https://github.com/w3c/svgwg/issues/1212 
for SVG2 spec part, which lacks clarification.

[Radar WebKit Bug Importer]

<rdar://problem/190994871>