Offer mouse drawing when the mouse reaches for the toolbar

[marcosqlbi]

1.2.0 left Mouse drawing discoverable only in Preferences, which is the one place someone who does not know the feature exists will not look. The signal that they might want it is already in the application: picking a tool from the toolbar with the mouse. A pen user reaches for the palette with the pen, so a mouse arriving there is someone whose next stroke is going to disappoint them.

The dialog

MouseModeOfferWindow — "Draw with a mouse?", an unchecked Don't show me this again, and Enable mouse mode / Cancel.

The offer is a question, not a confirmation, and the two answers are not interchangeable: one changes what the left button means. So:

• Neither button is IsDefault or IsCancel, and Enter is swallowed. A focused non-default Button responding to Enter is version- and style-dependent, so rather than rely on it not firing, Window_PreviewKeyDown marks every Enter handled — it tunnels from the Window before any button's KeyDown — and invokes a button only when Keyboard.FocusedElement is one of ours. Enter with nothing focused, or with the checkbox focused, does nothing and leaves the dialog open.
• Escape closes, because dismissing is always safe.
• Nothing is focused on open. Keyboard.Focus(this) in Loaded, so the first Tab reaches the checkbox rather than a primed button — otherwise the checkbox takes focus and a stray Space answers the wrong question.

Three judgment calls

• Once a session. Prompting on every toolbar click would be intolerable; prompting once ever would lose someone not ready to decide. A Cancel answers this session, the checkbox answers every one after it.
• The offer is queued at background priority, not shown from the click handler, so the click first does what it came to do. The tool is selected, and the dialog then explains why it may not behave as expected.
• A SuggestMouseMode setting (settings version 13 → 14) with a Preferences entry under Input, because "don't show again" is otherwise a one-way door. WarnWhenNoDigitizer sets the precedent.

Enable mouse mode sets MouseMode.On rather than WhenNoDigitizer, because the offer can only have appeared where the automatic default already decided not to.

Decision 24 records all of it. Version is 1.2.1.

Verification

Build clean, Core smoke tests pass including the settings round-trip and the 13 → 14 upgrade. Those tests are UI-free, so the dialog's behaviour is not covered by them; the README gains a second validation list under Mouse drawing validation, and every item on it is about a key doing nothing — the part a later change is most likely to undo.

A slow-shutdown regression was investigated against this branch and did not reproduce on the maintainer's machine. A clean launch-and-close measures 85–358 ms. No change was made for it.

🤖 Generated with Claude Code