This strikes me as very much a 21st century sort of comment, which looks at the "what" and totally fails to consider the "why" and the historical context.

> The thick borders took up valuable screen space and weren’t necessary.

Define "necessary". I think you are not considering why they were present in the historical context.

> They weren’t present prior to Mac OS 8

Yes, but there are reasons for that which I will go into.

> and they weren’t present after Mac OS 9.

There was no "after"; MacOS 9 (no space) was the last version. It was replaced.

> You might consider the era of thick borders as a 5-year blip on the timeline from 1997 to 2002.

Which fails to consider what happened in that timeframe.

Up to System 7.x you could only resize a MacOS (note, again, no space; that was important) window from the bottom right corner, where there was quite a big widget for this sole purpose... but in a brilliant bit of UI design, it was at the intersection of the vertical and horizontal scrollbars. Continuing either of them into the space past the end of the other would imply priority and that was a bad thing; the classic MacOS UI thought about this.

Examples: pics of MacOS 1.1.

https://guidebookgallery.org/screenshots/macos11

Now consider what happened in 1996. Apple was in big trouble, bought NeXT, and Steve Job came back. The primary reason was to replace MacOS with Mac OS. (Note: that's why the space is important. MacOS = classic; Mac OS = OS X.)

Jobs cancelled Copland, the planned MacOS 8, and directed the internal Apple team to start salvaging what could be taken from Copland into what was really MacOS 7.7 or something, renaming it to MacOS 8 in order to make it look big and important.

It wasn't; it was UI tweaks and stuff. E.g. the multithreaded Finder from Copland, and the Appearance control panel that allows skinning of the OS, which MacOS couldn't do before.

(All this while the new NeXT team are porting OpenStep to PowerPC and building a VM to run Classic in, stuff that has no customer impact or benefit yet.

Important point #1: this is adding a lot of customisability to the MacOS UI that wasn't there before. This is not some minor trivial point of graphical design.

Important point #2: this is Jobs aping a Microsoft tactic.

Windows 98 is the same timeframe. Win98 is the same basic OS as 95, but with UI tweaks. Why? Because NT 4 is late, and not ready for consumers, but also, because at the time, MS is fighting the US DoJ over monopoly claims, because MS is bundling IE with Windows.

To fight this MS rewrites bits of the Windows Explorer in IE. Gaining, oh hey look, what a coincidence, a multithreaded Explorer, because window contents are rendered in HTML... which means it gets a selling point to upsell W95 customers to W98.

Apple borrows the adaptable UI stuff from Copland and backports it to MacOS 7.

Result: now you can resize a window from any side, like Windows. Jobs comes back and Apple starts "borrowing" ideas from Windows UI and MS GUIs, something pre-Jobs Apple wouldn't do, and only fair as Microsoft "borrowed" so much from Apple.

So how do you show that a window can be resized from any side, not just from the bottom right corner? Answer: you put big fat draggable window borders on it, just like Windows has.

Pics: https://guidebookgallery.org/screenshots/macos80

That's why those borders were there.

Because Apple was recycling tech from its own, very expensive, failed new-OS development project, so that it could:

[1] offer UI tweaks that [a] enabled it to upsell customers an OS facelift and [b] showed that it had learned both UI and business methods from MS.

[2] as a byproduct kill the Mac clones as that only covered System 7

[3] find a use for the hundreds of millions of dollars it spent on Copland

[4] show people it could adapt and survive and sell new stuff while the NeXT team worked on Rhapsody, which would in time become Mac OS X.

In other words, there are very good strong reasons by those windows got big fat borders, important reasons that helped to save the company.

Second lump of history you failed to consider.

Why didn't Mac OS X (note, with a space), have those?

Two reasons.

[1] Because NeXTstep didn't have fat window borders, and Mac OS X is NeXTstep. NeXTstep only let you resize from bottom left or bottom right, and to do that, it had a big fat bottom window border with market bits at the left and right end to show you where to grab.

Pics: https://guidebookgallery.org/screenshots/openstep42

And why didn't NeXTstep use the bottom right corner? Because NeXTstep doesn't put scroll bars on the right. It puts them on the left.

Why? Because in the late 1980s when Jobs started NeXT, Apple had recently sued Digital Research over GEM, because GEM looked too like MacOS (no space), and it won, and PC GEM was crippled as a result.

So the little startup company founded by the departed leader of the hostile litigious one does things as differently as it can so it can't get sued by the CEO's former employers. Or by Microsoft.

So, no menu bar, menus stack up on the left.

So, scroll bars are somewhere else.

So, windows aren't resizable by the edges and don't have fat window edges.

Aside:

Fun fact: Motif had those fat window edges and resizing from all corners and all edges too, because the design of Motif was licensed from Microsoft by the OSF. This is also why Motif menu bars are inside the window, like on MS Windows. Because back then MS was trying to be Not Like Apple, and Apple wasn't like anyone by default, but both sued anyone who copied their designs.

And that's also why almost all Linux desktops today are recycling the same tired old ideas. Because since the dawn of GUIs on xNix, it's been under the influence of Apple and Microsoft designs.

Sun did interesting stuff in OpenWindows and OpenLook but it wasn't really "open" despite the name and it's gone now. Damned shame.

SGI did some, in other areas, in IRIX. Also not really open. Also gone now. Also a shame, although there is the Maxx desktop, but nobody cares because everyone else did an end-run around it and the industry moved on. By 1993 it was a little throwaway gag in _Jurassic Park_ -- "hey, this is Unix, I know this!"

End of aside.

Mac OS X dropped that clunky bottom bar, because when the litigious company that sues copiers owns you you don't need to worry. So, scrollbars move back to the right. Menu bars go on the top screen edge, like MacOS and GEM and AmigaOS, and where Fitt's Law of ergonomics says they are easy to hit.

Result: Rhasody copies the MacOS 8 design.

Pics: https://guidebookgallery.org/screenshots/rhapsodydr2

And then Mac OS X one point zero, sold as Mac OS X 10.0, uses 3d shading to show the edges of windows, because by the 21st century you could assume the display was in 24-bit colour and could do stuff like that.

Pics: https://guidebookgallery.org/screenshots/macosx100

And truecolour icons and things, and shading everywhere, because when you control the hardware you can assume stuff like this and show it as the only way of showing window edges.

Fun fact: we early adopters of Mac OS X used little utilities to turn off the window shadows because it sapped performance on the low-powered old Macs we were using to run this stuff.

But oh hey look, the window corner widget has come back because we need to tempt Mac users onto the new OS, too.

Because Jobs _thought_ of stuff like this. How can I use any of the failed OS project my company rendered obsolete? How can I salvage some of all that wasted R&D budget? How can I sell interim releases? How can I make MacOS (no space) a little bit more familiar to Windows users? MS is making money selling small UI innovations to Windows customers, so how can I do that too?

There are good solid reasons for all this stuff, reasons totally driven by business models and IP ownership and tech of the time.

It wasn't accidental. It wasn't a blip. It was all for very good reasons, all of which you just skipped over with your rash assumption that it was a glitch of late-1990s design cosmetics.