"Attach Node Script" dialog box spawns slightly off-center

[ghost]

Operating system or device, Godot version
Linux Mint 18.3 Sylvia x64, Godot 3.0 f3ad142

Issue description:
The dialog box always seems to appear too far down and to the right:

[vnen]

This happens for me only the first time the dialog is open. The next time I open the dialog is in the center.

[ghost]

@vnen Good observation.

[kidrigger]

I called popup_centered() twice in a row instead of just once and it started to align to center. I'm trying to trace the reason why the second call centers the dialog.

[akien-mga]

See #16069, which is a later duplicate but has some more debugging details, so closing this one in favour of #16069.

[ghost]

This behaviour still occurs as of dd2cd06.

However, it only happens when adding a script from the Scene dock; creating a script from within the Script editor (File > New Script...) has the dialog centered correctly the first time. I narrowed it down to this line:

godot/editor/plugins/script_editor_plugin.cpp

Line 971 in dd2cd06

script_create_dialog->popup_centered(Size2(300, 300) * EDSCALE);

The Scene dock's dialog is popped up without any size information:

godot/editor/scene_tree_dock.cpp

Line 394 in dd2cd06

script_create_dialog->popup_centered();

Adding it fixes the problem. I have no idea why, though. Is it worth making a PR for this, or is a proper fix desired?