fix(textinput): correct placeholder layout constraints (px vs DIP) and no-op NaN fontSize guard

[FaithfulAudio]

Forward-port of #16303 (which targets 0.83-stable) to main, per @acoates-ms:

yeah, we should get a main check-in too otherwise it wont make it into future releases.

Both defects are present on main today, in WindowsTextInputComponentView::CreatePlaceholderLayout().

1. Placeholder layout constraints were fed physical pixels, not DIPs

m_imgWidth/m_imgHeight are physical pixels (frame * pointScaleFactor), but facebook::react::LayoutConstraints are expressed in DIPs. The placeholder was therefore laid out inside a box pointScaleFactorx too large, so it measured and positioned at a different height than the typed text — visible as the placeholder sitting at a different baseline than the value the user types, and worsening as scale increases.

-  constraints.maximumSize.width = static_cast<FLOAT>(m_imgWidth);
-  constraints.maximumSize.height = static_cast<FLOAT>(m_imgHeight);
+  constraints.maximumSize.width = static_cast<FLOAT>(m_imgWidth) / m_layoutMetrics.pointScaleFactor;
+  constraints.maximumSize.height = static_cast<FLOAT>(m_imgHeight) / m_layoutMetrics.pointScaleFactor;

This version divides directly, with no != 0 guard — per review feedback on #16303. LayoutMetrics::pointScaleFactor has a default member initializer of 1.0 and EmptyLayoutMetrics only designates .frame, so even the pre-layout sentinel inherits 1.0; there is no path here where it is 0.

2. The NaN fontSize guard was a no-op

The guard evaluated defaultTextAttributes().fontSize as a discarded expression statement rather than assigning it, so a placeholder with no explicit fontSize never actually picked up the default:

   if (std::isnan(props.textAttributes.fontSize)) {
-    facebook::react::TextAttributes::defaultTextAttributes().fontSize;
+    textAttributes.fontSize = facebook::react::TextAttributes::defaultTextAttributes().fontSize;
   }

Testing

Verified on the 0.83-stable version of this change (#16303): compiled into a framework source build (binary-gated) and re-probed in a production app at 250% scale — no regression, the NaN-fontSize guard now actually assigns (a no-fontSize placeholder renders at the default size), and placeholder-vs-typed are pixel-comparable at that geometry. Evidence attached to #16303.

This main port is the identical change applied to the current main source; it has not been separately run on-device, so CI is the gate here.

Related: #16303 (0.83-stable).

Microsoft Reviewers: Open in CodeFlow

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