To reproduce:

1. Create a new project.
2. Create 3d Cube, just to better see what happens.
3. Go to the "Game window", but do not press play.
4. Hold ctrl and try to zoom in with the scroll wheel.

Expected result: screen gets stretched horizontally, and zooms when you don't scroll. (The way zoom works when shift is pressed, but with shift it get stretched vertically).
Actual result: screen gets stretched horizontally, and jumps back to normal, without zooming.

Zoom in "Game window" added in 5.4.

Reproducible with: 5.4.0f1, 5.5.0f1, 5.6.0a3