Why does my screen have a bright or dark patch?
Several distinct faults produce patches, and identifying which one you have determines whether it is fixable, whether it will spread, and whether it is worth repairing.
Backlight bleed and clouding — on LCD panels. Uneven illumination, worst at edges and corners, visible on dark content. Caused by light escaping around the panel edges, frequently from assembly pressure or a slightly warped frame.
Present from new to some degree on most LCDs, and a matter of tolerance rather than failure unless severe. It does not worsen significantly, and it is not repairable short of panel replacement.
Pressure damage. A patch of distorted colour, frequently with a distinct shape matching whatever pressed on it. Caused by physical force on the panel — a laptop lid closed on an object, a thumb on the screen, or transport pressure. Permanent.
Screen burn-in (image retention) — on OLED. A faint permanent ghost of static content — status bars, navigation buttons, channel logos, interface elements — caused by uneven ageing of the pixels that have been driven hardest. It accumulates and is irreversible. Mitigations such as pixel shifting and logo dimming slow it.
Temporary image retention on some LCDs looks similar and fades, which distinguishes it.
Liquid damage. A dark or discoloured spreading patch, frequently with visible edges, that grows over days. A spreading patch is the diagnostic signature, and there is no repair short of replacement.
Failed backlight zone — on full-array local dimming displays, one zone dark or stuck bright.
A failing panel connection, producing bands or a dark region that changes when the screen is flexed slightly — which is the one worth having looked at, since it is sometimes a cable rather than a panel.
Stuck versus dead pixels: a stuck pixel shows a fixed colour and occasionally recovers; a dead one is black and does not.
How to identify yours:
Display a full white, then full black, then solid colour image. Each fault is visible on different backgrounds, and this takes seconds.
Does it move when you change content? A ghost of previous content is retention or burn-in.
Is it growing? Growth means liquid damage.
Does it change with pressure or angle?
Check warranty terms, since panel uniformity is frequently excluded below a threshold.