Notebooks: Track the Active Cell For Selection

[dhruvisompura]

Addresses #9848 #10280 #10281 #9992 which are all active cell/focus/selection related bugs and #10470 which just updates the clear output icon to match the icons we use elsewhere in the IDE.

This is a pretty large PR that addresses a number of selection behavior bugs and some selection/hover styling requests as described in the issues above. The root cause for many of these issues was that we didn't really treat a single cell as the one with focus in multi-select scenarios.

Following the commits to understand the changes would be an easy way to understand what each change is doing.

Fixed:

• Updated "Clear Output" action bar button icon
• Cell action bar only shows up for active cell in single select and multi-select scenarios
• The active cell now always shows the blue focus ring around the cell to help differentiate it from a selected cell.
• Cell actions should now work in reference to the active cell
• Hovering over a cell that is not the active cell or a selected cell will now have a grey border hover state. The cell action bar will NOT show up when hovering over a cell since we only show the action bar for the active cell.
• "+ Code" and "+ Markdown" action bar buttons now insert the cell after the active cell and make it the new active cell
• You can escape a multi-select scenario by pressing Escape
• Multi-select scenarios now support de-selecting cells via Shift+ArrowUp or Shift+ArrowDown. Previously the selection would always grow the selection but now multi-selection has an "anchor" which is the active cell. You can only increase a selection of cells in one direction. Moving in the opposite direction will decrease the cell selection. Ex: Given a file with 5 cells (cell 0 to cell 4), if the active cell is cell 2, Shift+ArrowUp twice means selected cells are cell 0, cell 1, cell 2. Pressing Shift+ArrowDown twice makes the selection be cell 2.

There are probably other things that may have been fixed with this change but the above were intentionally fixed. I'd normally include videos but there is A LOT that has changed and capturing it all in a few videos isn't totally possible.

Not Fixed:

• Cut and delete operations for multi-select scenarios
  • This will be addressed in a follow up as part of Notebook: Cut operation with multi-cell selection removes extra cell and only pastes the incorrect cell #10116
• Cmd+click to multi-select non-contiguous cells and Shift+click for contiguous selection
  • This has not been directly addressed but it does look cmd+click is working for copying cells. Shift+click works the same as cmd+click which is incorrect. All of these issues will be addressed in Notebook: Cut operation with multi-cell selection removes extra cell and only pastes the incorrect cell #10116

Hover State

Screen.Recording.2025-11-14.at.11.59.04.AM.mov

Basic Multi-Select + Active Cell Keyboard Navigation

Screen.Recording.2025-11-14.at.12.00.22.PM.mov

Release Notes

New Features

• N/A

Bug Fixes

• N/A

QA Notes

@:positron-notebooks

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:positron-notebooks

^{readme  valid tags}

[dhruvisompura]

Removed the onMenuStateChange prop because its not needed in NotebookCellActionBar.tsx anymore.

[dhruvisompura]

A good chunk of the styles in here that don't have a comment about being for hover behavior were changes needed so the focus border shows up on the active cell.

[seeM]

Epic PR! 🎉 Feels much nicer and smoother when growing/shrinking the selection, escaping out of a multi selection, and hovering over cells

Separate but related bug: Shift+Enter should execute all selected cells in the order they appear (not order they're selected), but currently executes only the active cell.

Screen.Recording.2025-11-17.at.14.03.23.mov

Super minor nit for discussion: Something about how the gap between the cell code bar and output bar fits into the cell border doesn't feel right:

[seeM]

Nit

Suggested change

	public readonly isActive = observableValue<boolean, void>('cellIsActive', false);
	public readonly isActive = observableValue('cellIsActive', false);