fix(app): backport rendering improvements (#1580)

fluxer_app/src/features/theme/layout/RemFromPx.ts

@@ -2,7 +2,32 @@
 
 export const REM_BASE_PX = 16;
 
+class NonFinitePixelValueError extends Error {
+	public constructor(value: number) {
+		super(`Pixel value must be finite: ${value}`);
+		this.name = 'NonFinitePixelValueError';
+	}
+}
+
 export function remFromPx(px: number): `${number}rem` {
+	if (!Number.isFinite(px)) {
+		throw new NonFinitePixelValueError(px);
+	}
 	const rounded = Math.round((px / REM_BASE_PX) * 1e5) / 1e5;
 	return `${rounded}rem`;
 }
+
+export function getRemScaleForDocument(ownerDocument: Document | null): number {
+	if (ownerDocument == null) {
+		return 1;
+	}
+	const ownerWindow = ownerDocument.defaultView;
+	if (ownerWindow == null) {
+		return 1;
+	}
+	const rootFontSize = Number.parseFloat(ownerWindow.getComputedStyle(ownerDocument.documentElement).fontSize);
+	if (!Number.isFinite(rootFontSize) || rootFontSize <= 0) {
+		return 1;
+	}
+	return rootFontSize / REM_BASE_PX;
+}