While it is possible to hard-code widget positions, with the Container-based layout method the values in that file actually get completely ignored & recalculated on the fly.

The fact that the values end up in the file at all is something I consider a bug/implementation flaw.

In Godot a UI Control has an "anchor" setting for each of left/right/top/down margin which affects where the margin value is measured from in relation to the bounding box. So (hand-waving a bit here), e.g. for, say a margin value of 10px, for the left margin an anchor value of 0 means "inset 10px from the left margin of bounding box" and with anchor value of 0.5 means "move left 10px from center of bounding box" and with anchor value of 1 it calculates relative to the right-most margin. (But for right margin an anchor value of 0 means "calculate relative to right margin of bounding box, and, somewhat confusingly, I think requires the value to be negative to "inset".)

What this means in practice, is that if you use an anchor value of 0 for all margins, even when you resize an element you don't need to adjust any of the margin values.

Unfortunately Godot (and Containers--even though they re-calculate all the margins themselves) default to an anchor value of 0 for left margin and 1 for right margin which means the right margin is a non-zero value & gets re-calculated & updated/save every time you adjust the layout.

In general the layout system works pretty well & is flexible but it does have some slightly confusing aspects (and in the past at least some implementation bugs) and is somewhat difficult to explain in text. :) (And it would definitely benefit from even more detailed documentation--than what it currently has--in terms of the "sizing flags" that determine how multiple Controls occupy/share the space available.)

So, in short: Once set up correctly, Godot UI layouts generally adapt well to different text-widths, font sizes and window dimensions.