Casted on Youtube for the July 1v1 tournament.

Semifinals(W) (Group L), Game 3

It's not you, Forever. Spring engine doesn't handle double-click properly. I actually figured out that double-click misbehaviour is what happened to you when splitting the videos and mentioned your explanation in the description.

What it should do: When clicking on a particular unit twice in quick succession, select all of type on screen; otherwise when clicking on a unit, select only that unit

What it does: When clicking twice in quick succession, and the second click lands on a unit, select all of type on screen.

The key difference is that there is no check of whether or not the first click was on the same unit, or even on a unit at all. As a result, if you are clicking around, miss the first selection click, and then rapidly get the second click on the unit you want, it will select all units of that type on screen. It happens to me all the time with workers and it's a PITA to re-assign them to their specific jobs.