Explore supporting replace while markdown cells are in preview mode

[rebornix]

Related feature #177542

[rebornix]

Feedbacks received from yesterday's notebook sync and UX channels

• Find on markdown content and preview based on their cell editing state
  • It's weird to call this "Hybrid Markdown", especially when it's next to "Markdown Source" and "Rendered Markdown"
  • Unnatural that the markdown section buttons are Toggles other than checkboxes
  • There isn't a way to enable search on both Markdown Source and Preview, how about then we use this state (both of them checked) to represent the new mode (searching based on editing state)?
• Replace turns all markdown cells into editing state
  • Jo: Can we turn the markdown cells into editing state only when users type in the replace input box or when users use mouse to manually click the buttons
  • Michael: Can we only turn the cells into editing mode if they have matches

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At the same time, we compared how other notebook frontends handle this problem

• Jupyter Lab
  • CMD+F now opens its own find widget
  • It turns cells which contain matches into editing mode
  • Cells switch back to preview mode if there is no match (it always goes back to preview mode regardless of its initial state)
• Colab
  • CMD+F opens browser find widget, it searches what's visible/rendered (but sometimes stuck in cell editors)
  • CMD+H opens builtin find widget, which is positioned in the sidebar and renders like our Search View
    • It turns active cell into editing mode
    • Other cells containing find matches stay in preview mode, but it's not an issue as users can see all matches preview on the sidebar