I doubt this is a good time to consider such changes, but maybe for 2.81. This patch fixes a problem that most people are probably not aware of and is therefore difficult to explain...
To resize editor windows we need to place our mouse in the gaps between them and drag. But sometimes it is more difficult to position your mouse correctly than you would expect, especially if using a tablet pen. This is because the borders that you see are not the same size as the actual gaps.
The following table lists the difference between the gaps and the borders. You will see that not only are they always different, but that difference varies in unpredictable ways.
This patch makes it so that the positions of the editors and the gaps between them exactly match the borders that are drawn. This way you can always be assured that you can drag while your mouse is anywhere in the gap.
I tried to make it so that default sizes are as close as possible as they are now so most people would not notice a difference. Headers are currently defined as 26 pixels tall even though we see them to be 24 (because 2 pixels are covered). So this patch changes the header height definition to 24 pixels and removes an adjustment for this.
Doing this does bring us very close to being able to have borders widths as a user preference. This would be ideal as width of the borders should not really be tied to line width. You might be perfectly happy with how it all looks but then get a graphics tablet that requires wider borders. Unfortunately doing this would have to wait. The border line drawing code ties the border radius to the width of the border. So this would have to be change so we can keep radius (tied to scale) but vary only border width.