Image loading is the least portable thing in your shared UI
Decode, cache and memory pressure are platform concerns, and the composable that hides them decides your scroll performance.
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A feature flag read on four platforms needs one source of truth and four caches
Flags that differ per platform at the same moment produce bug reports nobody can reproduce.
A dependency that resolves in commonMain and only exists on Android
Gradle metadata decides which variant each target gets. A library with no native artifact fails at link, not at resolution.
Shared navigation has no back stack owner, and that is the hard part
Android has a system back gesture with an owner. iOS has a navigation controller. A shared back stack answers to both.
KSP runs once per target, and your generated code can disagree with itself
A processor sees one compilation at a time. Generating into commonMain is a race you win by accident.
TextFieldState is a document, and edits are transactions on it
TextFieldState models text, selection and composition together. Editing through its transaction API preserves undo history and cursor position in ways string replacement cannot, which matters for any non-trivial input.
There is no File in commonMain, and that is a feature
wasm has no filesystem and iOS has a sandbox with policy. An interface forces you to name what you need.
Hashing a password in shared code is the one thing you should not share
Each platform has a vetted crypto provider. A pure-Kotlin implementation has none of their review.
Throwing across the bridge loses your exception type
Swift sees NSError with a wrapped message, so a typed catch becomes a string match.
There is no UUID in the Kotlin standard library, and the fixes differ per platform
Random is seeded differently per target and is not secure anywhere. An id generator is a security decision.
Your list rows are different heights because text measurement is honest
Text height depends on content, font and scale, which makes uniform list rows harder than they look. Line-break strategies, hyphenation and baseline-relative padding are the tools that solve it without fixed heights.
@Preview is Android-only, and your shared UI has no preview at all
The annotation lives in an Android artifact, so commonMain cannot use it — and that shapes how you write composables.