Open Radar

 
Area:
AppKit

Summary:
NSScrollView asks its NSScrollers to `drawKnob()` when you scroll; it also queries `rect(for: .knob)`. But the actual drawing is not affected until you command-tab out of the app or scroll to the edges of the document to make the knobs shrink. This means there's no programmatic way to initiate a change in the drawing reliably as the scroll view seems to cache the drawing result, even though the reported rectangle changes.

Steps to Reproduce:
- add a text view inside a NSScrollView to the app
- create a custom NSScroller subclass and use it as the vertical scroller
- create a toggle that affects the style of the custom scroller, e.g. change the `rect(for: .knob)` size and draw something different than before in `drawKnob()`

See minimal sample project: https://github.com/DivineDominion/bugreport_NSScrollerKnob

Expected Results:
The knob changes its appearance and size the next time it becomes visible.

Observed Results:
The knob stays as it was until you command-tab out of the app or move the knob to the edges of the scroll view to make it shrink.

Version:
10.12.5 (16F73)

Notes:
Minimal sample project: https://github.com/DivineDominion/bugreport_NSScrollerKnob

Configuration:
Using a Magic Trackpad and having auto-hiding scrollers enabled.