Distinguish an unexpected hold from normal key repeat
A held-key list is built from keydown and keyup events received by this page. It is not a global view of every key the operating system considers pressed. Keys already held before the page became active may not appear until another event arrives.
Press and release a key while watching the list. Its code should appear on press and leave on release. A hold longer than three seconds is highlighted to draw attention, but a long intentional hold is normal and does not establish a stuck switch.
Automatic repeat starts after the delay set by your operating system. It produces repeated keydown events while you hold a key. A high repeat count alone is not a fault; the important comparison is whether repeating continues after you physically let go.
Switching windows or using an OS shortcut can hide the corresponding keyup from a browser. This test clears held state on focus loss to avoid carrying those stale holds forward. If a key appears held unexpectedly, refocus, press and release it, then compare in another application.
A physically stuck modifier can change how other keys behave, but Sticky Keys and remapping can also latch modifier behavior without a mechanical problem. Check those settings before concluding that a highlighted Ctrl, Alt or Shift needs repair.
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