If I understand correctly, a quick-and-dirty fix (not that I would submit it) would be to just re-add the tabIndex to the sidebar controls? Or would there be something else preventing me from tabbing over its controls? If I remember correctly then I couldn’t tab anywhere except for the composer, so I guess you took the easy way and just removed all tabIndex rules except for the few elements in there?
Update 1
Actually… A fix should be fairly simple using this line of code:
document.getElementsByClassName("name")[numberIndicatingSidebarEntry].click()
I can easily traverse through the sidebar nodes that way.
Update 2
I managed to get somewhat convenient keyboard-navigation on the sidebar by injecting this code in chrome debugger:
__counterForSidebarFolders = 0;
__nameList = document.getElementsByClassName('name')
document.addEventListener("keydown", event => {
moveDown = false;
moveUp = false;
if (event.ctrlKey && event.shiftKey && event.keyCode == 9) moveUp = true;
else if (event.ctrlKey && event.keyCode == 9) moveDown = true;
console.log(moveDown);
console.log(moveUp);
//Only change counter if an actual move-event happens
if (moveDown || moveUp) {
//The below code is necessary to always start navigation at the already selected folder
[...document.getElementsByClassName('name')].forEach((item, index) => {
selectedItem = document.getElementsByClassName("selected item");
selectedItemIndex = moveDown ? selectedItem.length - 1 : 0;
if (item == selectedItem[selectedItemIndex].getElementsByClassName("name")[0])
__counterForSidebarFolders = index;
}
)
}
//The actual keyboard-navigation...
if(moveUp && __counterForSidebarFolders > -1) {
__counterForSidebarFolders--
__nameList[__counterForSidebarFolders].click()
}
else if(moveDown && __counterForSidebarFolders < __nameList.length)
{
__counterForSidebarFolders++
__nameList[__counterForSidebarFolders].click()
}
})
Just in case someone else needs a quickfix… It works by pressing ctrl+tab and ctrl+shift+tab but is easy to adapt to custom hotkeys. I’m currently looking into how to permanently modify the electron package (as I really doubt the code above fullfills the quality standards of Mailspring, so I’ll not attempt a pull request 
Update 3
Took me some time, but I figured out the solution was to edit chrome-extension.js inside the electron.asar file. I added this code at the bottom:
app.on('browser-window-created', function (event, bWindow) {
bWindow.webContents.executeJavaScript(`
__counterForSidebarFolders = 0;
__nameList = document.getElementsByClassName('name');
document.addEventListener("keydown", event => {
moveDown = false;
moveUp = false;
if (event.ctrlKey && event.shiftKey && event.keyCode == 9) moveUp = true;
else if (event.ctrlKey && event.keyCode == 9) moveDown = true;
console.log(moveDown);
console.log(moveUp);
//Only change counter if an actual move-event happens
if (moveDown || moveUp) {
//The below code is necessary to always start navigation at the already selected folder
[...document.getElementsByClassName('name')].forEach((item, index) => {
selectedItem = document.getElementsByClassName("selected item");
selectedItemIndex = moveDown ? selectedItem.length - 1 : 0;
if (item == selectedItem[selectedItemIndex].getElementsByClassName("name")[0])
__counterForSidebarFolders = index;
}
)
}
//The actual keyboard-navigation...
if(moveUp && __counterForSidebarFolders > -1) {
__counterForSidebarFolders--;
__nameList[__counterForSidebarFolders].click();
}
else if(moveDown && __counterForSidebarFolders < __nameList.length)
{
__counterForSidebarFolders++;
__nameList[__counterForSidebarFolders].click();
}
})
`)
})
For convencience I’ll add the re-packaged electron.asar (of the current version, will likely be outdated soon though), just replace it with mine. The location depends on your OS, on linux a simple whereis mailspring did the job. On Windows I don’t know, but you can check in your Program Files and Application Data folders or follow this guide:
sidebar-keyboard-navigation.zip
Update 4
I just noticed a bug. In case you have more than two mail accounts connected to mailspring, and you attempt to skip through their corresponding subfolders in the “All Accounts” view (ctrl+1), my code would always jump from the first to the last acount… You can probably avoid this by switching to the account-specific views (ctrl+2, ctrl+3, …), at least that issue didn’t occur for me in there. Subfolders worked just fine.
Actually I didn’t know about those shortcuts before, but they come in very handy… I mostly wrote the JS code from above to have a way to switch accounts via keyboard. Guess using those combined with the hotkeys for drafts, sent, inbox, … is an acceptable workaround though.
Update 5
Instead of browser-window-focus I used browser-window-created. Otherwise multiple EventListener would be registered each time the window gets focused on, leading to messed up jumps of multiple controls. Also fixed in the zip!
Now you just should be aware about the fact that you’ll have to re-add the changes made to the .asar file each time you update. If you really want to you can probably write up a bash script for that job though…
Update 6
I wrote a bash file myself. It’s intended mostly for personal usage but in case Mailspring’s electron.asar archive is in /usr/share/mailspring/resources it should work for you as well:
mailspring-update-fix.zip
Just execute it after each update and you should be fine. The chrome-extension.js file containing the actual fix is hidden, in case you wonder.
(Originally posted by a now deleted user on GitHub.)