Reading issue
sveltejs / svelte · Issue No. 18531
Setting a new value for a key that already exists in a SvelteMap re-runs effects that only read the key set (keys(), has() misses), as long as that key was never read individually. Such a key has no source yet, so set() takes the new-key branch and increments the version signal unconditionally even though the key set didn't change. In practice {#each [...map.keys()] as key} re-renders on every value write into the map.
Fix incoming.
Click the button and watch the console, keys: logs on every click although the key set is always ['a'].
annoyance
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The full analysis for this issue is still being assembled. Until then, the description above and the thread on GitHub are the most reliable context.