Tab and shift+tab doesn't work as expected

Several times a day I use shift-tab to try to move focus from the message list to the Mailboxes pane, and every time I'm annoyed that it doesn't work and I have to use the mouse/trackpad instead.

In general, pressing the tab key—with or without the shift key–moves focus, but not always as one would expect.

For one, focus seems to be "lost" for two presses of tab. Steps to reproduce:

• Click on a message in the message list. Press tab. Focus moves away from message list, but not to any visible place. Pressing arrow keys does nothing, pressing letter keys produces error sound.
• Press tab again. Same result as in previous step.
• Press tab again. Focus moves to search box.
• Press tab again. Focus moves to Mailboxes pane.
• Press tab again. Focus moves back to message in message list.

Shift-tab does the same thing, but doesn't move focus to the Mailboxes pane:

• Click on a message in the message list. Press shift+tab. Focus moves to search box, instead of to Mailboxes pane, as expected.
• Press shift+tab again. Focus disappears, as in previous examples.
• Press shift+tab again. Same result.
• Press shift+tab again. Focus moves to message in message list.

If one deselects the message, so the message preview pane is empty, tab focus has four steps:

• Mailboxes pane
• Message list, but invisible until you press down arrow (to focus on first message) or up arrow (to focus on last message).
• Unknown
• Search box

And with no message selected, shift-tab also never focuses on Mailboxes pane, and has only three steps:

• Search box
• Unknown
• Message list, but invisible until you press down arrow (to focus on first message) or up arrow (to focus on last message).