Sounds silly, but I’ve clicked something and animation timeline in my UMG widget zoomed in very much. How to zoom out? I’ve tried many key-scrollwheel combinations, but nothing.
Restarting editor helped there, zoom returns to default value after that. It’s still a ‘placeholder’ solution though 
If you use your scroll wheel while inside the dark grey portion of the timeline (where the keyframe numbers are displayed) it zooms in and out.
for newer versions of UE4, you need to hold Ctrl + Scroll with your mouse wheel WHILE the cursor is on the dark gray area (stated in the answer above), additionally, hold Ctrl + Left click and drag on the dark gray area to highlight how much of the timeline you want to zoom in.
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