Try the built-in option first
After taking a screenshot, check whether the preview offers a Full Page tab. Availability depends on the app and content. When it appears, it is usually the fastest option. When it does not, capture the screen in sections.
Capture a clean sequence
Take a screenshot, scroll so that roughly one quarter of the previous screen remains visible, and capture again. Repeat until the whole conversation or page is covered. A larger overlap is easier to match than two screenshots that touch at a single line.
Merge in Safari or another modern browser
Open TallMerge, select the screenshots from Photos, confirm their order, and run overlap detection. Use manual crop values for sticky navigation bars or timestamps that change between captures. Export to PNG for sharp text, or JPEG/WebP when file size matters more.
Keep mobile memory in mind
A very tall original-resolution image can exceed the browser canvas limit on a phone. Choose a 1080 or 1440 pixel maximum width, split an extremely long conversation into two files, or finish the largest jobs on a desktop browser.