Drag the border between two columns to set their widths in 1.16.0

Umut, with a card of an icon next to a text as the example: "bei Rows die
Breite ändern, am besten per Drag and Drop im Editor". Rows only ever had
equal columns - both the classic `columns` token and the free `gridColumns`
produce `repeat(n, 1fr)`.

A row now carries the free style `columnWidths`, a list of shares such as
`2 1` or `66.7 33.3`, per device and without a new document schema.
`StyleCompiler::columnTracks()` turns it into `grid-template-columns`,
pads or cuts the shares to the number of columns, and gives a device whose
layout stacks the columns a single track, because the desktop rule carries
an id and would otherwise outrank the class that stacks a phone.

In the editor the border between two side-by-side columns of the selected
row is a handle: dragging restyles the row inline while the pointer moves
and writes one undo step on release; the arrow keys nudge by five points.
The design panel shows the same value as the text field "Spaltenbreiten".
The Playwright editor check drags the handle with the real mouse.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/assets/canvas-overlay.js

@@ -35,7 +35,7 @@ const TOOLS = [
   { action: 'delete', glyph: '✕', title: label => `${label} löschen`, danger: true },
 ];
 
-export function createCanvasOverlay({ stage, frame, getScale, getDocument, getSelected, onSelect, onAction, onInsert, onDrop }) {
+export function createCanvasOverlay({ stage, frame, getScale, getDocument, getSelected, onSelect, onAction, onInsert, onDrop, onResize = () => {} }) {
   const layer = document.createElement('div');
   layer.className = 'canvas-overlay';
   const hover = element('div', 'ov-box ov-hover');
@@ -205,6 +205,7 @@ export function createCanvasOverlay({ stage, frame, getScale, getDocument, getSe
     // The toolbar is placed last because it has to keep clear of the upper plus sign, and only
     // now is it settled whether there is one.
     positionToolbar(selectedBox, Boolean(sibling));
+    placeResizers();
     const hoverBox = hovered && hovered !== selectedId ? rectOf(hovered) : null;
     if (place(hover, hoverBox)) paintLabel(hoverLabel, hovered);
     if (dragging) {
@@ -213,6 +214,124 @@ export function createCanvasOverlay({ stage, frame, getScale, getDocument, getSe
     }
   }
 
+  // -------------------------------------------------------------------------------------------
+  // Column widths. Umut: "am besten per Drag and Drop im Editor".
+  //
+  // The border between two columns of the selected row is a handle. Dragging it moves share from
+  // one neighbour to the other; the row is restyled inline while the pointer moves, and the
+  // result is written to the document once, when the pointer is released. The arrow keys do the
+  // same in steps of five, so the widths are reachable without a mouse.
+  // -------------------------------------------------------------------------------------------
+
+  const resizers = [];
+  let focusResizer = null;
+
+  /** The row whose borders can be dragged: the selected row, or the row around the selected column. */
+  function resizableRow() {
+    const item = getSelected() ? locate(getDocument().blocks, getSelected()) : null;
+    if (!item) return null;
+    if (item.block.type === 'row') return item.block;
+    return item.parent?.type === 'row' ? item.parent : null;
+  }
+
+  /** The measured widths of the columns as shares of their sum - the truth a drag starts from. */
+  function sharesOf(rowId) {
+    const kids = childElements(rowId);
+    const total = kids.reduce((sum, kid) => sum + kid.box.width, 0);
+    return total > 0 ? kids.map(kid => kid.box.width / total * 100) : null;
+  }
+
+  /** Moves `delta` points of share from the right neighbour to the left one; neither may vanish. */
+  function shift(shares, index, delta) {
+    const pair = shares[index] + shares[index + 1];
+    const least = Math.min(10, pair / 2);
+    const left = Math.max(least, Math.min(pair - least, shares[index] + delta));
+    const next = shares.slice();
+    next[index] = left;
+    next[index + 1] = pair - left;
+    return next;
+  }
+
+  const tenth = share => Math.round(share * 10) / 10;
+  const formatShares = shares => shares.map(share => String(tenth(share))).join(' ');
+  const tracksOf = shares => shares.map(share => `minmax(0,${tenth(share)}fr)`).join(' ');
+
+  function placeResizers() {
+    const row = dragging ? null : resizableRow();
+    const kids = row ? childElements(row.id) : [];
+    // Stacked columns - a phone, or a single-column layout - have no border to drag.
+    const sideBySide = kids.length > 1 && kids.every((kid, index) => index === 0 || kid.box.left >= kids[index - 1].box.right - 1);
+    const wanted = sideBySide ? kids.length - 1 : 0;
+    while (resizers.length > wanted) resizers.pop().remove();
+    while (resizers.length < wanted) resizers.push(resizeButton());
+    const width = 14 / getScale();
+    resizers.forEach((button, index) => {
+      const left = kids[index].box, right = kids[index + 1].box;
+      button.dataset.row = row.id;
+      button.dataset.index = String(index);
+      const label = `Breite zwischen Spalte ${index + 1} und ${index + 2} ändern`;
+      button.setAttribute('aria-label', label);
+      button.title = `${label}: ziehen oder Pfeiltasten`;
+      button.style.left = `${(left.right + right.left) / 2 - width / 2}px`;
+      button.style.top = `${Math.min(left.top, right.top)}px`;
+      button.style.width = `${width}px`;
+      button.style.height = `${Math.max(left.bottom, right.bottom) - Math.min(left.top, right.top)}px`;
+      if (focusResizer && focusResizer.row === row.id && focusResizer.index === index) { button.focus(); focusResizer = null; }
+    });
+  }
+
+  function resizeButton() {
+    const button = document.createElement('button');
+    button.type = 'button';
+    button.className = 'ov-resize';
+    button.addEventListener('pointerdown', startResize);
+    button.addEventListener('keydown', event => {
+      const step = event.key === 'ArrowLeft' ? -5 : event.key === 'ArrowRight' ? 5 : 0;
+      if (!step) return;
+      event.preventDefault();
+      const rowId = button.dataset.row, index = Number(button.dataset.index);
+      const shares = sharesOf(rowId);
+      if (!shares) return;
+      focusResizer = { row: rowId, index };
+      onResize(rowId, formatShares(shift(shares, index, step)));
+    });
+    layer.append(button);
+    return button;
+  }
+
+  function startResize(event) {
+    if (event.button !== 0) return;
+    const button = event.currentTarget;
+    const rowId = button.dataset.row, index = Number(button.dataset.index);
+    const start = sharesOf(rowId);
+    const node = doc()?.getElementById(rowId);
+    if (!start || !node) return;
+    event.preventDefault();
+    button.setPointerCapture(event.pointerId);
+    layer.classList.add('is-resizing');
+    // Pointer pixels are editor pixels; the shares live in the frame's scale, on the summed
+    // width of the columns without the gaps between them.
+    const content = childElements(rowId).reduce((sum, kid) => sum + kid.box.width, 0);
+    const startX = event.clientX;
+    let current = start;
+    const move = moved => {
+      current = shift(start, index, (moved.clientX - startX) / getScale() / content * 100);
+      node.style.gridTemplateColumns = tracksOf(current);
+      sync();
+    };
+    const finish = () => {
+      button.removeEventListener('pointermove', move);
+      button.removeEventListener('pointerup', finish);
+      button.removeEventListener('pointercancel', finish);
+      layer.classList.remove('is-resizing');
+      if (formatShares(current) !== formatShares(start)) onResize(rowId, formatShares(current));
+      else sync();
+    };
+    button.addEventListener('pointermove', move);
+    button.addEventListener('pointerup', finish);
+    button.addEventListener('pointercancel', finish);
+  }
+
   // -------------------------------------------------------------------------------------------
   // Dropping.
   // -------------------------------------------------------------------------------------------