fix(compat): avoid mutating user-provided style objects when appending px

[JSap0914]

Bug

handleDomVNode in compat/src/render.js mutates the style object supplied by the caller when appending px to numeric values:

// before
if (i === "style" && typeof value === "object") {
    for (let key in value) {
        if (typeof value[key] === "number" && !IS_NON_DIMENSIONAL.test(key)) {
            value[key] += "px"; // ← mutates props.style in place
        }
    }
}

value is a direct reference to props[i] (the original object passed by the user). After this loop the caller's object is permanently modified—margin: 10 becomes margin: "10px"—which is an unexpected side effect.

Fix

Shallow-copy the style object before the loop so mutations stay local to normalizedProps:

value = assign({}, value);

The px-append behaviour and the rendered output are unchanged; only the caller's object is now left untouched.

Verification

npx vitest run compat/test/browser/render.test.jsx

New test: "should not mutate the original style object when appending px" — this test failed before the fix (style.margin was mutated to "10px") and passes after it.

All 46 existing compat-render tests continue to pass; full suite: 1242 passed, 12 skipped across 101 test files.

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[marvinhagemeister]

The PR description says:

(...) mutates the style object supplied by the caller when appending px to numeric values

which hints that it's only a problem when px is appended. The change here though is broader. It also applies when no px is appended too. That seems wasteful in terms of performance that we always pay the cost of cloning an object for that too.

[marvinhagemeister]

Let's simplify this:

Suggested change

	const style = { margin: 10, opacity: 0.5 };
	const originalMargin = style.margin;
	const originalOpacity = style.opacity;
	render(<div style={style} />, scratch);
	expect(style.margin).to.equal(
	originalMargin,
	'style.margin should not be mutated'
	);
	expect(style.opacity).to.equal(
	originalOpacity,
	'style.opacity should not be mutated'
	);
	const style = { margin: 10, opacity: 0.5 };
	render(<div style={style} />, scratch);
	expect(style).to.equal({ margin: 10, opacity: 0.5 });

[JSap0914]

The clone is now lazy — only runs the first time a numeric value actually needs appended, so style objects with no such properties pay zero clone cost.