Wouldn't they at least take the opportunity to have a free hand to consolidate their Silesian position?
Or use the ensuing Lithuanian war for some concessions? Say, demanding a reiteration of Kazimierz's renouncement of the Polish claim to Silesia?
Might they be tempted at having a go at Brandenburg, given their free hand?
In the case of a William of Poland and Austria, wouldn't it bother them about being outflanked, given that Poland had only recently (under Kazimierz) renounced its claim on Silesia? Might they be tempted to enter the war on Lithuania's side, or to persuade the TO not to enter it?
But it was already consolidated, Polish affairs had nothing to do with Silesia in 1385.
Brandenburg was ruled by a cousin of Bohemian king, why should Bohemians bother with attacking it? They've done it during the rule of Wenceslas's father, Charles IV of Luxembourg. And Wenceslas wasn't particularily a great king. He was a drunkard and he lost Imperial crown and was imprisoned by his own Bohemian subjects due to his ineptitude.
How they would be outflanked? William didn't rule over whole Austrian lands, he had his share, but his principality wasn't bordering Bohemian lands. No, they wouldn't be bothered, Bohemians had nothing against William's claim. Bohemia will be a perfect neutral state in the war - they don't have any reason and any political will to interfere in it. And TO wasn't even considering not entering war with a Lithuania. They were sort of allied to Jogaila, before he gained Polish crown (but better say that they forced him to sign a treaty giving TO a lot of gains) and if Vytautas dared to break his precedessor's promises than TO wouldn't hesitate to attack Lithuania and any protest of Wenceslas wouldn't stop it. In fact, giving any political suport, let alone entering the war on Lithuanian side will be a final blow to Wenceslaus himself. TO had it's fair share of supporters in HRE - German princelings were sending their younger sons to TO's land to not have to maintain them at home. If TO turned against Wenceslas it'd mean an end of his rule in HRE. To counterweigh this he'd need to have a military support for a significant foreign power. His brother, Sigismund wasn't able to accomplish this, because he had to fight to maintain Hungarian crown for himself and his wife and this fight wasn't going well for him at that stage - he lost Hungarian crown to Charles of Durazzo, and even after his assasination he had to fight his supporters and give a lot of crown properties to nobility as a lien. That's why Wenceslas was asking (IOTL) Jogaila for help and was even willing to renounciate his claims to Silesia in favor of Jogaila (Jogaila only declined, because he was depressed after death of his first wife). Having said so, I kind of disagree with
@alexmilman 's vision of Lithuanian-Russian tsardom. First of all, you need to notice that position of Lithuania in 1385 was not exactly the same as it was during Witold's reigns - Witold had a significant share of Polish armed troops in his campaings and he didn't hesitate to use them. For example, in the battle of Vorskla among Witold's commanders was a man called Spytko II of Melsztyn (Spytko II Melsztyński), which was one of highest-ranked Polish aristocrats of that time. Witold also took Smoleńsk in 1404 with help of a Polish troops. So we cannot use position of Lithuania from end of OTL Witold's rule - when he had Polish troops to help him expand in the East and Muscovy was ruled by an underaged prince being also Witold's grandson as a role model of ATL 1385 Witold's Lithuania. While Witold in 1430 was recognized a "grand protector" of Muscovy, Jogaila in 1385 was required by Muscovites to marry daughter of a Grand Prince and call himself an "younger brother" of said Grand Prince - and in Ruthenian/Russian lands it meant a form of subordinacy of one ruler to another. And Witold was reducing lesser princes to mere officials - and it's something which wouldn't be as smooth as it was without Poland. In addition, if Jogaila was assasinated in 1385, that Skirigaila (Jogaila's younger brother, closest advisor and prince of Kiev) would assume the throne for himself. And he was already Orthodox, so if Witold thought about taking the throne in TTL 1385 he'd need to place himself as a leader of a faction opposed to Orthodoxy with possible conversion to Catholicism in exchange of Polish and Order's help in taking the throne.