SpriteFrames editor forces window to be wider/taller than screen

[Jerferson4]

Godot version: Godot_v3.0-rc3_win64

OS/device including version: Windows 10 64bits, screen size 1024*768

Issue description: will it's just a simple and harmless issue: when I create an animated sprite and go to 'spector' and set 'new spriteframes' at 'frames' and then open the spriteframes box it just cause a part the right boxes (the ones below the play button) to go off the screen, and even when I close the spriteframes box the right boxes continue hiding some parts. I aways have to minimize and then maximize the window to solve temporarely.

Steps to reproduce:

Minimal reproduction project:

[Calinou]

This issue has the same cause as #26835.

[akien-mga]

Consolidating in #26835.