Sheet tabs: drag the handle at the scrollbar's left edge to give the tabs more or less room

The sheet tabs took up to 60% of the tab bar and the horizontal
scrollbar the rest, and nothing between them could be dragged. The
gap at the scrollbar's left edge is now a handle: it shows the resize
cursor, and dragging it right gives the tabs more of the bar, until the
scrollbar is 100 px wide. After the window is narrowed, the scrollbar
is still 100 px wide. Dragging it left gives the scrollbar more, down
to the New sheet button. Room the tabs do not fill stays empty, and
New sheet stays after the last tab.

Each open workbook keeps its own share while it is open. The share is
not saved in the file, and a workbook starts at the default.

docs/ui-parity.md

@@ -1,6 +1,6 @@
 # UI parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-23 · **Change:** minor (New Formatting Rule's formula box takes references clicked or dragged on the grid, #2348; Page Up and Ctrl+Up into frozen rows, Ctrl+Left and Alt+Page Up into frozen columns bring the scrolled area back; Auto Fill Options button after a left-drag of the fill handle (#1403); Show Outline Symbols, Ctrl+8; a too-wide number in a repeat-fill format such as Accounting shows #### instead of losing its leading digits; Esc and Enter close a data validation alert over the cell editor; Ctrl+Enter fill gives each cell the automatic number format typing gives it; keytip letters typed with Alt held, #1846; Number Stored as Text indicator and Convert to Number) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-24 · **Change:** minor (dragging the handle between the sheet tabs and the horizontal scrollbar resizes the tabs; New Formatting Rule's formula box takes references clicked or dragged on the grid, #2348; Page Up and Ctrl+Up into frozen rows, Ctrl+Left and Alt+Page Up into frozen columns bring the scrolled area back; Auto Fill Options button after a left-drag of the fill handle (#1403); Show Outline Symbols, Ctrl+8; a too-wide number in a repeat-fill format such as Accounting shows #### instead of losing its leading digits; Esc and Enter close a data validation alert over the cell editor; Ctrl+Enter fill gives each cell the automatic number format typing gives it; keytip letters typed with Alt held, #1846; Number Stored as Text indicator and Convert to Number) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Interaction, controls and feel, against Excel. Part of [target-app-parity.md](target-app-parity.md).
 **UI/UX fidelity: ~55% ready, 50–80 h** (estimated). Ribbon command coverage is measured
@@ -68,7 +68,7 @@ this logic, with one undo step.
 | Move and resize charts, pictures and shapes | charts can't be resized (#190) |
 | Context menus on row/column headers, sheet tabs, charts, shapes | headers and tabs done; objects partial |
 | Quick Analysis (Ctrl+Q) | missing |
-| Smooth scrolling, scrollbars | done; vertical scrollbar added 10-09 (#73); horizontal scrollbar in the tab bar (60/40 split, drag, track click pages) 10-13 (#176, #840); the tabs/scrollbar split can't be dragged yet |
+| Smooth scrolling, scrollbars | done; vertical scrollbar added 10-09 (#73); horizontal scrollbar in the tab bar (60/40 split, drag, track click pages) 10-13 (#176, #840); dragging the handle at the scrollbar's left edge gives the tabs more or less of the bar; each workbook keeps its share while it is open (not saved in the file) |
 | Text overflow into empty neighbours, wrap, rotation, shrink to fit | done |
 
 ## Keyboard
@@ -178,6 +178,7 @@ scrollbar bugs 2–4 · pixel/feel pass against observed Excel 7–12.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-24 | minor | Sheet tabs: the handle at the horizontal scrollbar's left edge shows the resize cursor, and dragging it moves the boundary, as in Excel ("Hover the mouse pointer at the edge of the scrollbar until you see the double-headed arrow"). Dragging it right gives the tabs more of the bar, until the scrollbar is 100 px wide (the scrollbar stays 100 px wide after the window is narrowed); dragging it left gives the scrollbar more, down to the New sheet button. Room the tabs don't fill stays empty, with New sheet after the last tab. Each open workbook keeps its own share; a workbook starts at the default (tabs up to 60%) |
 | 2026-10-23 | minor | Conditional Formatting ▸ New Rule, formula: clicking a cell on the grid writes its absolute reference at the formula box's caret (`=$B$2`, an empty box starts with `=`), a drag writes a range (`$B$2:$C$4`) and a second click replaces the reference just written, as in Excel ("Selecting cells on the worksheet inserts absolute cell references"). The selection the rule applies to doesn't move, and the box keeps the keyboard. After an operand (`=A1>5`) the click does nothing (#2348) |
 | 2026-10-23 | minor | Frozen panes: Page Up, Ctrl+Up and Shift variants that move from the scrolled rows into the frozen ones bring the scrolled rows back to the top, so the rows in between are seen again (Page Up from far down used to end on row 1 with the view still a page down); Ctrl+Left and Alt+Page Up do the same for frozen columns. Other ways of selecting a frozen cell (click, Find, Go To, Undo, formula pointing) leave the view where it is |
 | 2026-10-23 | minor | After a left-drag of the fill handle an Auto Fill Options button sits below the filled cells; its menu redoes the fill as Copy Cells, Fill Series, Fill Formatting Only or Fill Without Formatting (undoes the drag's fill, then `edit.autoFill` in that mode). It goes away when the workbook changes or the selection moves (#1403) |