Fix FPS-dependent movement acceleration caused by ExtraMouseSample()#1949
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bjorn-martinsson wants to merge 1 commit into
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Fix FPS-dependent movement acceleration caused by ExtraMouseSample()#1949bjorn-martinsson wants to merge 1 commit into
bjorn-martinsson wants to merge 1 commit into
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This was referenced Jun 4, 2026
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This PR fixes the FPS-dependent movement acceleration bug described in ValveSoftware/Source-1-Games#8045.
The root cause is that
CInput::ExtraMouseSample()consumes movement key impulse state before it is processed byCInput::CreateMove(). SinceCInput::ExtraMouseSample()may run a different number of times depending on FPS, acceleration becomes both FPS-dependent and nondeterministic.This patch snapshots movement key state before
CInput::ExtraMouseSample()and restores it afterwards, preventing movement input from being consumed prematurely.In addition to fixing ordinary movement input, this also fixes the issue where tick taps (pressing and releasing a movement key within the same tick) randomly fail depending on FPS.
This is intentionally a minimal fix. A more complete solution would likely require redesigning parts of the Source 1 input system, which would be significantly more invasive and carry a greater risk of regressions.
The patch has been tested in TF2. Since the issue exists in shared Source 1 input code, it should also apply to other Source 1 games.
One consequence of this fix is that first-tick acceleration will consistently follow the original Source behavior of applying half acceleration on the first movement tick. Players who are accustomed to playing at high FPS may perceive this as a reduction in responsiveness, since the bug often caused full acceleration to be applied instead.
A second PR (#1950) builds on top of this one and changes the first-tick acceleration from half acceleration to full acceleration. The first PR can be merged independently. The follow-up PR cannot.