The old measurement used tabbar.clientWidth which includes horizontal
padding (~2em / ~30px total), causing an over-allocation of the same
amount. Flex gap between tabs (0.2em) was also not counted in the
accumulated offsetWidth, adding another ~20px of error. Together these
caused the algorithm to leave too many tabs visible, overflowing the
bar and pushing the ⋮ button off the right edge of the screen.
Replace the clientWidth + cumulative-offsetWidth approach with
getBoundingClientRect: the cutoff is the tabbar's actual right edge
minus its right padding, minus a reserve for the overflow wrapper.
Individual tabs are overflowed once their right edge exceeds the
cutoff; subsequent tabs are overflowed unconditionally to keep the
visible set contiguous and left-anchored.
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