[HMR] `swapStyle()` API does not account for stylesheet collisions

[nolanlawson]

In the current implementation of swapStyle, unrendering stylesheets does not work correctly if there is a chance collision of CSS content between two separate .css files.

For example, let's say we have two identical CSS files:

/* a.css */
div { color: red }

/* b.css */
div { color: red }

And a third, different CSS file:

/* c.css */
div { color: blue }

And say we render two components, one with A and the other with B. Now we run:

swapStyle(a, c)

This will correctly replace A with C in the first component. However, it will incorrectly also replace B with C in the second component, since the contents for A and B have happened to collide.

Note this only occurs in cases where it's possible for the exact CSS content to collide in the same target DocumentOrShadowRoot, e.g.:

• top-level light DOM components with non-scoped style
• native shadow DOM components with non-scoped style

The reason for this is that style scoping / synthetic shadow scoping defeats the collision by appending generated attributes/classes to the CSS selectors inside the CSS content.

Minimal repro: nolanlawson@43e3d9b (passes for scoped light, fails for native shadow and global light)