The sidebar, folder tree and outline

Show the files in a folder and the headings of the open file on the left, move between them with the mouse or keys, and hide the sidebar when you don't need it.

The sidebar sits on the left of the window, under the top bar and the tabs. It has two parts, Files and Outline. Each window has its own sidebar.

Show or hide it

Click the Sidebar button at the left end of the top bar.
Or press Cmd+Shift+E on Mac, Ctrl+Shift+E on Windows and Linux.
Or choose Toggle Sidebar in the command palette.

A new window starts with the sidebar hidden. Show it with any of the ways above. Once you show or hide it, that window remembers your choice. Drag the sidebar's right edge to make it wider or narrower (180 to 480 pixels, and never more than half the window). With the edge focused, the Left and Right arrow keys move it in small steps. In a narrow window the sidebar lies over the page instead of pushing it aside.

Click Files or Outline to fold that part away; click it again to bring it back.

Whether the sidebar is shown, its width, and the folder you picked come back for each window after you quit and start Sashmark again.

Files

Files lists one folder: sub-folders first, then Markdown files (.md and .markdown), each group sorted by name. Hidden names that start with a dot, such as .git, and every other kind of file are left out. A link to another folder or file is shown with a small arrow; clicking it does nothing, so it never opens or expands.

Until you pick a folder, the list shows the folder of the file in the active tab, and it follows you when you switch to a tab in another folder. A New Tab has no file, so the list shows only the Open Folder… button.
To pick a folder yourself, click Open Folder… at the top of Files, choose File > Open Folder… (Mac and Linux), or choose Open Folder… in the command palette. The folder you pick stays for that window until you pick another one.
Click a folder to open or close it. Its contents are read only when you open it.
Large folders appear in small groups of rows so you can keep working while the list loads.
Click a Markdown file to open it. If it is already open in a tab, Sashmark switches to that tab; if another window has it, that window comes to the front.
The file in the active tab is marked in the list.

When files are added, removed or renamed in an open folder, the list updates by itself, usually within a second. A folder with no Markdown files says No Markdown files. A folder Sashmark can't read, or one that was deleted, says Can't read this folder. On a Mac or Linux, so does a folder you picked that was then renamed or moved while open, the next time its list is read or you click a file in it (that click opens nothing): choose Open Folder… again to pick it at its new place. If the folder moves between your click and the moment the file loads, the file does not open (Sashmark says it was moved or deleted); it never shows a file that took its place. A tab you opened from the sidebar stays tied to that folder: once the folder is renamed, moved or replaced, the tab no longer reloads or saves, and says the file was moved or deleted; your edits stay in the tab. Open the file again from its new place. If Sashmark can't watch a folder for changes, it says so instead of showing a list that would not update; open that folder again (or choose Try again for the top folder) to retry. Sashmark watches up to 64 open folders per window; past that, the folder you just opened says Too many folders open: close one to see changes here. If the folder you picked is gone when Sashmark starts again, the list goes back to following the active file, and you see one message saying so.

The sidebar only reads the names in a folder. It never changes, moves, renames or deletes anything, and it never shows files outside the folder you picked.

Outline

Outline lists the headings of the file in the active tab, nested by level. Click a heading, or press Enter on it, to scroll the page to it; this works for two headings with the same text too. The heading you are reading is marked as you scroll. The outline updates as you type, when the file changes on disk, and when you switch tabs. A file without headings says No headings.

Keys

Up and Down move through the list; Home and End go to the first and last row.
Right opens a folder, or moves to its first item; Left closes a folder, or moves to the folder that holds the item.
Enter opens the file, or jumps to the heading.
Each list is one stop for Tab.
Escape takes you back to the page.

In high contrast

With Windows High Contrast, macOS Increase Contrast, or a Linux high-contrast theme, the rows, the marked row, the focus ring and the sidebar's edge use the system's own colours.