Platform: Windows 10 (1703)
Problem 1: The zoom percentage textbox always steals focus of the newest image tab, unless there'is only one tab
Reproduce:
1. Check Option>General>"Allow only one instance of XnView"
2. Open image A, then open image B
3. Press "+" button on keyboard
Expectation:
Image B zoom in
Actual result:
Nothing happened except you typed a "+" into the zoom percentage textbox
Demo:

Problem 2: Your focus can't "escape" once the zoom percentage textbox had it
Reproduce:
1. Open image A
2. Click in the zoom percentage textbox
3. Click on image A (or anywhere of the application), then press "+" button on keyboard
Expectation:
Image A zoom in
Actual result:
Nothing happened except you typed a "+" into the zoom percentage textbox