Right, I'm not saying that the GUI was rendered using the DOS box-drawing characters, i.e. the monitor's built-in codepage 437 text mode; nor were they using a monospace bitmap font of codepage 437.

What I'm saying, is that I'm pretty sure that a lot of the Windows 1.0 GUI was drawn by emulating the "technique" TUIs use to draw box-drawing characters, but on a framebuffer and with a custom (monospace) bitmap font:

1. create any-and-all graphical detail you need on screen, by repurposing the "leftover" parts of the bitmap font you're using to draw control-label text to the screen, adding a set of additional, custom symbol-drawing elements, which are all 'characters' of that bitmap font, and so all stuck being the same size+shape as the label text;

2. create a graphical "monospace text" drawing primitive, that takes as input a pair of buffers representing the text itself, and its hardware-text-mode-alike per-character drawing attributes (i.e. FG + BG color);

3. implement your OS widget library almost entirely in terms of calls into that graphical "monospace text" drawing primitive, passing a static data-section buffer holding the positional+attribute data for your box-drawing character.

(For example, look at the drive icons in the File Manager. Those are just clearly just "text" composed of four drawing-element characters from the bitmap font, like this: [-=-]. So the whole drive-chooser area can be drawn with a single text-draw command, passing a string like "A[-=-] C[-=-] C: \WINDOWS".)

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You might say "but look at any screenshot of Windows 1.0 — the first thing you'll notice is that the menu-item labels in each window's menu bar are offset by a half-character-width horizontally! And modal dialogs are, in their entirety, offset by a half-character horizontally and vertically! How's that possible?"

Well, the GUI "monospace text"-drawing primitive might be drawing a grid of characters to the framebuffer; but it's not drawing them to an imaginary grid on the framebuffer. It accepts an arbitrary pixel offset for where it should start to draw the block of monospace text.

So, with that in mind, the algorithm for drawing the menu bar very likely has two passes:

1. render some box-drawing characters representing the "background" of the menu (i.e. yellow with a black border)

2. render a layer of regular non-box-drawing characters on top, offset by +4px on the X axis, in "black on transparent", representing the menu-item labels.

It's pretty clear (to me, at least) how a drawing algorithm like this could be a natural evolution and outgrowth of a TUI: first, replace the TUI's backing text buffer with a framebuffer, and the character-plotting calls with draw calls to a "draw monospace text character at emulated-grid position on framebuffer" primitive; then refactoring the draw calls to use a window-local coordinate basis; and only then adding the ability to draw anything other than monospace characters—but gradually, starting only with defined grid-snapped "rich graphical content" regions within windows (sort of like how Windows today has "Direct3D drawing surface" regions); and then going back and gradually enhancing the GUI widgets with little flourishes like draw offsets.

Within the Windows 1.0 codebase, I would bet money that—at least in some previous revision in early development—there was probably a #define flag for whether the "framebuffer driver" was enabled; and that all these windowing-system and common-controls drawing algorithms were written in a "hybrid" way where, instead of one TUI-based and an entirely-distinct framebuffer-based implementation, the framebuffer-based implementation is just #ifdef'ed ornamentation on top of the base TUI implementation.