fix: avoid applying data-md-button styles twice after the toolbar is moved

[kwy404]

Root cause

MarkdownToolbarElement.connectedCallback() adds its keydown and click listeners every time the element is connected. The click listener is the same function each time, so the browser ignores the duplicate, but the keydown listener is a new keydown(applyFromToolbar) wrapper on every call. Each time the toolbar is moved in the DOM (disconnected and connected again) it gains one more keydown listener. Pressing Enter or Space on a data-md-button element then applies the style more than once, and for toggling styles like headers or bold the second run undoes the first, so the key press seems to do nothing.

Fix

Add the two listeners once in the constructor, the same way MarkdownButtonElement already does. They live on the element itself, so they do not need to be removed or added again on disconnect and reconnect.

Test

Added a test that adds a <button data-md-button="header-6">, moves the toolbar, and presses Enter on the button. Without the fix the textarea ends up as |title| (the header is added and then removed) and the test fails; with the fix it is ###### |title| and the test passes. npm test (lint, build and the full Karma suite) passes: 144 tests.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused lifecycle fix is consistent with existing button behavior and includes appropriate regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Moves toolbar event listener registration to construction time, preventing duplicate keyboard handling after DOM moves.

Changes:

• Registers keyboard and click listeners once in the constructor.
• Adds regression coverage for moved toolbars.

File	Description
src/​index.ts	Prevents duplicate event listener registration.
test/​test.js	Tests keyboard activation after moving the toolbar.

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