Fix increaseNestingLevel/decreaseNestingLevel for multi-item list selections

[blshkv]

Bug

When multiple list items are selected and the user clicks the indent or
outdent toolbar button (or presses Tab / Shift+Tab), only the first
selected item changes its nesting level. All other selected items are ignored.

Root cause

Both increaseNestingLevel() and decreaseNestingLevel() in
src/trix/models/composition.js call this.getBlock(), which always returns
the single block at locationRange[0].index — the start of the selection.
They then call this.document.replaceBlock() exactly once and return.

// before — only acts on the first block
increaseNestingLevel() {
  const block = this.getBlock()   // ← always locationRange[0].index
  if (!block) return
  return this.setDocument(this.document.replaceBlock(block, block.increaseNestingLevel()))
}

Fix

Iterate from locationRange[0].index to locationRange[1].index, applying
replaceBlock() for each block and threading the updated document through the
loop. A single setDocument() call is made at the end.

// after — covers every block in the selection
increaseNestingLevel() {
  const locationRange = this.getLocationRange()
  if (!locationRange) return
  let document = this.document
  for (let i = locationRange[0].index; i <= locationRange[1].index; i++) {
    const block = document.getBlockAtIndex(i)
    if (block) document = document.replaceBlock(block, block.increaseNestingLevel())
  }
  return this.setDocument(document)
}

The same change is applied to decreaseNestingLevel().

Tests

Two regression tests are added to src/test/system/list_formatting_test.js:

• "increasing nesting level applies to all selected items" — creates a
  3-item bullet list, selects all three, clicks increaseNestingLevel, and
  asserts every block gained an extra nesting level.
• "decreasing nesting level applies to all selected items" — creates a
  3-item list where items 2 and 3 are nested, selects both, clicks
  decreaseNestingLevel, and asserts both items returned to the top level.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Extend list nesting (indent/outdent) actions to operate over the current multi-block selection instead of only the active block, and add system tests to validate the selection-wide behavior.

Changes:

• Update increaseNestingLevel / decreaseNestingLevel to iterate over the selection’s index range and apply the nesting transform per block.
• Add system tests asserting nesting changes apply across multiple selected list items.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/trix/models/composition.js	Applies nesting level changes across the selected range instead of a single block.
src/test/system/list_formatting_test.js	Adds system tests for selection-wide increase/decrease nesting actions.

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