1. For scrolling capture to work, you can only outline the area (object) that would ACTUALLY scroll, if you used scroll bars normally.
2. Initiate scrolling capture (default hot keys are Ctrl + Alt + PrtScrn, or select "scrolling window" from menu > Screen Capture>). Only position red border around actual scrolling area if you were to use a mouse wheel or grab the scroll bar slider (not stationary borders, toolbars).
The border of browser page or tabs bar don't scroll, so don't select those after initiating scrolling capture.
3. For entire browser page, have cursor over the up arrow of scroll bar (red outline around scrolling screen area, including scroll bar, but not screen border. Without moving cursor (red outline still correct position), click near the corner of the scrolling object. IMPORTANT: after scrolling starts, don't move cursor around or outside the scrolling object - it can stop scrolling.
4. Alternate method (still works in v4.20) - position mouse & red outline as before. In browser, with cursor over scroll bar up arrow, click in the scroll bar channel - on up arrow or slider. Drag cursor downward a bit. You don't have to drag to end of page. Once you click, DON'T move cursor around OR outside of scrollable object until scrolling is finished. Leave cursor still after L clicking.