Autofill a series by dragging the fill handle

Dragging the small square at the bottom-right of a selection down or right extends a detected pattern — a lone number or date increments by one step, two-or-more numbers/dates continue their observed step, a recognized weekday or month name advances through its list, and a formula's references shift exactly like a copy. Anything that isn't a recognized pattern repeats as a cycle, which is exactly a plain repeat for a single-cell source.

What it does

Select a cell (or a small range) and drag the little square at its bottom-right corner — the fill handle — down or right. TrueCalc extends the content as a detected series instead of just copying it:

A lone number increments by 1 each step (5 → 6, 7, 8, …) — Sheets' own well-known behavior, different from spreadsheet apps that repeat a single number by default.
A recognized weekday or month name (full or abbreviated — Jan, Mon, January, Monday) advances through its list, wrapping around (November → December → January).
A formula's relative references shift exactly like a copy/fill; $-anchored ones stay put.
Anything else repeats as-is — the same "nothing recognized, so don't invent a pattern" behavior every fill has always had.

The series is detected fresh from whatever's selected before you drag — select two or more cells to seed a richer pattern (two numbers continue their exact step; three-or-more continue the average step between them).

Try it

Verified behavior

Matches Google Sheets: In Google Sheets, dragging the fill handle (the small square at a selection's bottom-right corner) extends the selection's content as a detected series rather than a plain copy: a single number increments by 1 (Sheets' well-known divergence from Excel, which repeats a lone number by default), two-or-more numbers continue their observed step, a recognized weekday or month name (full or abbreviated) advances through its list and wraps around, and a formula's relative references shift exactly like a copy/fill while $-anchored ones stay pinned. Anything that doesn't match a recognized pattern repeats as-is. This daily-driver slice implements series-detection for a drag extending DOWN or RIGHT past the source's own bottom-right corner; Ctrl+D/Ctrl+R and a drag up/left of the source keep the pre-existing verbatim-repeat/ref-adjust-copy behavior unchanged. Per the oracle brief's disclosed tooling failure, every specific rule here is built from Sheets/Excel's own extremely well-established, long-stable autofill behavior rather than a fresh live observation this session.