qgis - Labels displaying with multiple bounding rectangles

While setting up labeling of a line (road) layer, the labels started displaying with multiple bounding rectangles along with the text.

After trying many different labeling configurations without getting only text to display, I was able to work around by abandoning the problem project and starting over with a new project, same data, same label configuration, and they started displaying correctly. So the problem seems to be with the project itself.

Easier to show than describe. Problem project -

Correct rendering -

Looks to me like the boxes are the tracks of the placement iterations of the labeling engine. Maybe I have bumbled into some debugging setting?

Using QGIS 2.18.16 running in Ubuntu 16.04.

Answer

You're right, it is a debugging setting. It can be handy for troubleshooting labeling issues.

It's next to the pull-down for the labeling list so you may have activated it by mistake. In my version, you have to specifically check on "show candidates" to enable this; perhaps in your version this is checked by default, so dismissing the dialog with OK enables the outlines..?

The icon to activate/deactivate is tucked away in the top-right corner of the layer properties > label dialog, shown circled below.