Desktop: Accessibility: Make Markdown toolbar scrollable when low on space

[personalizedrefrigerator]

Summary

This pull request makes the Markdown toolbar scrollable when content would otherwise be offscreen.

Screenshots

A window with width of 256px:

A larger window:

Relevant accessibility guidelines

From WCAG 2.2 SC 1.4.10 (emphasis added):

Content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions for:

• Vertical scrolling content at a width equivalent to 320 CSS pixels;
• Horizontal scrolling content at a height equivalent to 256 CSS pixels.

Except for parts of the content which require two-dimensional layout for usage or meaning.

Previously, functionality could be lost when displaying the note editor with a width of 320 CSS pixels, even with the sidebar and notes list hidden. This pull request allows users to continue to access items in the Markdown toolbar even on a small screen width.

Testing plan

1. Start Joplin.
2. Hide the sidebar and notes list.
3. Make Joplin's Window have a width of at most 320 CSS pixels.
4. Verify that a scrollbar is shown beneath the Markdown toolbar.
5. Drag the scrollbar to the right.
6. Click "toggle editors".
7. Verify that the Rich Text editor is visible.
8. Switch back to the Markdown editor.
9. Click the "insert drawing" button in the toolbar.
10. Verify that the drawing dialog is shown.