Three hot-path performance issues in autosize() caused forced synchronous reflows on every keystroke and unnecessary work on every mouse movement.

Changes

1. Batch DOM reads in sizeToFit

All layout reads (overflowOffset, getComputedStyle, container heights) now happen before any writes. Previously, textarea.style.maxHeight was written before getComputedStyle(container).height was read, forcing 2–3 synchronous layout recalculations per keystroke. The only unavoidable write-then-read is the height='auto' → scrollHeight measure pattern; it's now isolated with a comment explaining why.

Before (interleaved read/write/read):

textarea.style.maxHeight = `${maxHeight}px`          // write → layout dirty
container.style.height = getComputedStyle(container).height  // forced reflow
textarea.style.height = 'auto'                        // write
textarea.style.height = `${textarea.scrollHeight}px` // forced reflow

After (all reads first, then writes):

containerComputedHeight = getComputedStyle(container).height  // read (no dirty state)
// ... all other reads ...
textarea.style.maxHeight = `${maxHeight}px`           // write
container.style.height = containerComputedHeight      // write
textarea.style.height = 'auto'
const scrollHeight = textarea.scrollHeight            // single unavoidable reflow
textarea.style.height = `${scrollHeight + cachedBorderAddOn}px`

2. Cache getComputedStyle border/box-sizing values

cachedBorderAddOn (derived from borderTopWidth, borderBottomWidth, boxSizing) is computed once and reused across keystrokes. The library-tracked height string is also reused for the maxHeight calculation, avoiding a second getComputedStyle(textarea) call on steady-state keystrokes. Cache is invalidated on external resize and form reset.

3. Replace always-on mousemove with ResizeObserver

• Modern browsers: A ResizeObserver fires only when dimensions actually change. It compares textarea.style.height against the last library-written value; a mismatch means the user dragged the resize handle → sets isUserResized = true, clears maxHeight, then goes idle.
• Fallback (no ResizeObserver): The mousemove handler is gated behind mousedown/mouseup (with mouseup on document to handle pointer release outside the element), keeping it off the hot path during normal use.
• unsubscribe() calls cleanupResizeDetection(), which disconnects the observer or removes all registered listeners.