Upgrade to 1.0.51

Concerns: pushword/admin, pushword/admin-block-editor.

Incremental block editing

Sites with custom block tools or an undo normalizer: exports are cached by block data and tunes; unchanged history snapshots are reused. Keep exportToMarkdown() and normalizeBlockData() deterministic, with no side effects or dependency on invocation counts. No configuration change is required. See custom tools and undo normalization.

Group imports and narrow screens

Quoted show-more classes retain parentheses when importing Markdown. On narrow screens, an open outline overlays the editor instead of reducing its width. No configuration change is required.

Media picker

Selecting media while the picker is opening now closes the dialog reliably. No configuration change is required.