Wow, thanks Jargon - this is precisely the kind of feedback we really need 
Infinite loop - can you make some repro cases or post the assets? There may not be much I can do depending on what was involved, but I’ll be glad to take a look.
Standalone - not seeing that repro locally, do you have something I can test that has that issue?
Fullscreen size - It’s poorly named, need to change it to viewport size, but basically, if it’s non-zero, it will be the size of the widget in absolute mode when added to the viewport. Otherwise it will be sized based on the widgets desired size.
Open Level Crash - Will look into making that better. Thanks!
aspect ratio - Yup, that’s an in-progress feature 
Mostly just wanted to get the bones down on paper so people could see where I’m headed.
Using materials - Make sure the material is Emissive Only and is Marked for use in UI. See the screenshot here in the Material section, https://forums.unrealengine.com/showthread.php?25533-UMG-Documentation-for-4-4-Preview
Copy / Paste - Yeah I fixed a bunch of bugs there that should be in the next preview or final 4.4 build. The slot data not being preserved is a known problem, will definitely be fixing that soon. For the button case, try select the parent object, it has this issue right now where depending on what is selected, that’s the parent that it attempts to insert the paste into, but if you have a button selected with existing content, it can’t paste it into the button. I’ve thought about making it go up the hierarchy and find the first parent that will accept it. Not sure, needs some trail and error I think to find something expected/reasonable.
Undo - Yup buggy needs work
Checkboxes - Woops. Thanks, will get all that sorted out.
ListBoxes and ComboBoxes - Yeah they’re kind of a mess right now, the underlying slate object are very complicated so not sure yet how to best expose them to Blueprints. The Combobox is probably going to become a StringComboBox and then we’ll solve the more customizable combobox and listbox and tile view boxes in the future.
Dynamically resize or reposition the UI elements depending on the rendered resolution - there are future improvements coming in this area. I’ve got a Scale Box control I’ve created for the next version that can auto scale up or down but maintain aspect ratios of content. Good for text you need to fit a predefined area and background images of menus and such. For widgets added to the viewport you can adjust their Position, but not their anchor, I’ll work on improving that. Once The anchor are just normalized coordinates of the widget, so I can set something to 0.5, 0.5 to center it (assuming the alignment (aka pivot) is 0.5, 0.5 as well. Although you can currently do this now, but only inside of a widget, you’d need to throw down a Canvas, and then you could query and modify the state of a Widget’s Slot property at runtime. You just have to cast it to a CanvasPanelSlot, that’s the true own of all those Layout properties on a widget contained in a Canvas.
snap feature - Yup. You can see where I started that when you use the Cross arrow drag handle it will show snap margins, just doesn’t snap.
select multiple elements - Yup.
hide/show buttons in the treeview - Yup.
Data->Offsets - It needs a much better display in the canvas view, the problem is depending on your anchor points it means Position/Size, or it may mean Margin from anchors. The details panel needs to morph to show the right information of what they currently mean. Yeah I wouldn’t try to use them for Scale, scale is coming in another form, you’ll be able to apply a general transform to all widget soon as a render transform, so you can do translation, rotation and scale way easier.
Thanks again for all the feedback and bugs Jargon.
Cheers,
Nick