Today’s Question: I am suddenly noticing that Photoshop is showing a magenta “tail” that follows my brush strokes when I am painting, such as when cleaning up a layer mask. Is this a bug? Or what?
Tim’s Quick Answer: The line you’re referring to is the “leash”, which is a new feature related to automatic smoothing of your brush strokes in Photoshop CC 2018. This leash won’t appear if smoothing is disabled, but you can also turn off the leash altogether in Preferences.
More Detail: The latest update to Photoshop CC includes a smoothing feature for the various brush tools, which can help prevent a jagged appearance when your brush strokes aren’t especially steady.
The feature can be enabled (or disabled) using the “Show Brush Leash While Smoothing” checkbox found on the Cursors tab of the Preferences dialog. Turn this checkbox off if you never want to see the leash when using a brush-based tool (the Brush or Pencil tools, for example).
When the “Show Brush Leash While Smoothing”, you will see a line trailing your mouse cursor when you use one of the brush tools with the smoothing feature enabled. Any value above 0% for the Smoothing setting on the Options bar will cause the feature to be enabled. You can also adjust additional settings for brush stroke smoothing by clicking the settings popup to the right of the Smoothing setting.
The leash provides an indication of the direction and relative speed of each brush stroke, which enables you to better understand the behavior of the Smoothing feature and related controls.