Back in August 2018, Elon Musk casually announced on Twitter that he planned to take Tesla private at a price of $420 a share, a 20 percent premium at the time. He added that he had “funding secured.”

The announcement turned out to be much less secure than Mr. Musk had suggested, and it landed him in hot water with securities regulators, who asserted that he had misled investors. For more than a year after that, troubles seemed to mount for Mr. Musk and his electric-car company — including distribution challenges, a sales slump, quarterly losses, a liquidity scare and more legal problems for Mr. Musk.

All of that weighed heavily on Tesla’s share price, which fell as low as $177 in June.

But in recent months the company has seemed to turn a corner. Rising sales lifted Tesla to a profit in the third quarter, it unveiled a fourth car for its model line, and it completed a factory in China, a market of vast potential growth.

On Monday, its stock briefly reached a milestone, exceeding the $420 price that Mr. Musk once appeared to offer, before falling back. Mr. Musk’s Twitter feed featured a screen shot of a chart showing the intraday share price at $420.69 and two words: “stock art.”