the battle of Talas was this big turning point which ended Chinese expansion and influence in the region for a very long time

The Battle of Talas didn't end Chinese influence in the area. The An Lushan Rebellion, only a few years after Talas, ended Chinese control of the region. The Battle of Talas didn't even halt Chinese expansion in Central Asia - the Tang continued to expand during the short interval between Talas (751) and the An Lushan Rebellion (755-763).

It's possible that further fighting between the Abbasids and the Tang, which was likely to have happened if the An Lushan Rebellion hadn't occurred, would have affected Chinese expansion and influence, but the Battle of Talas would have only been an indecisive first battle in such a war. Talas only gains the appearance of a decisive battle due to the impact of the An Lushan Rebellion.

Did China really ignore their western border until the 1900's?

No, the Qing expanded far to the west in the 18th century. For a concise summary of Qing expansion, see this map showing the years when Qing control was established: https://en.wikipedia.org/wiki/Qing_dynasty#/media/File:Qing_Empire_circa_1820_EN.svg

The An Lushan Rebellion left the Tang in no condition to reestablish control over the west. After the Tang, the Song were engaged in ongoing warfare for the control of northern China, against the Liao/Khitans, the Jin/Jurchens, and the Mongols so, again, no expansion to the west. After the collapse of Mongol rule, Ming expansion to the northwest was minor, so we only see major expansion to the west mid-Qing (in the 17th century, the Qing were fighting to control China itself, with both the initial conquest and the Three Feudatories Rebellion, so their main expansion into Central Asia was in the 18th century).

For a thorough coverage of Qing expansion to the west, see Peter C. Perdue, China Marches West: The Qing Conquest of Central Eurasia, Belknap Press, 2010.