The ViewSonic XG2401 supports HDMI 1.4. I will now demonstrate it operating at 1920 × 1080 @ 144 Hz over HDMI. These tests are performed with an NVIDIA GeForce GTX 780 Ti, which also only supports up to HDMI 1.4.
Display Settings Demonstration
These settings show the XG2401 connected via HDMI on both ends at 1920 × 1080 @ 144 Hz with full RGB color. These settings are available out of the box without requiring any overclocking/custom resolutions. 1080p 144 Hz was in fact selected by default when the monitor was connected over HDMI for the first time, I didn't even need to set it to 144 Hz manually.
Timing Parameters and EDID
For 1920 × 1080 @ 144 Hz, ViewSonic has decided to define a set of custom timing parameters, with an effective resolution of 2026 × 1157 or a pixel rate of 337.0 Mpx/s, just barely within the 340 Mpx/s maximum of HDMI 1.4. Curiously, when connected via DisplayPort, the monitor uses slightly different parameters defined by the standardized CVT-R2 formula, 2000 × 1157 or 333.2 Mpx/s, which would also fall within the 340 Mpx/s limit of HDMI 1.4. However, these timings are not used for the HDMI connections for some reason.
The EDID reports a maximum pixel clock of 340 Mpx/s, the highest allowed by HDMI 1.4. The 1080p 144 Hz format is defined within the CTA-861 extension block.
The EDID is the same on both of the XG2401's HDMI ports, and 1080p 144 Hz works on both ports.
Of course, it is possible that the monitor is simply skipping frames, or failing to truly operate at 144 Hz in some other way. Some form of verification would be desirable.
Verification by Oscilloscope
This is measured using a Keysight EDUX1002A oscilloscope and a Texas Instruments TSL14S light-to-voltage converter. A pattern of alternating black and white frames was generated by the blurbusters flicker test (https://testufo.com/flicker). Since oscilloscopes are designed for measuring oscillating waveforms, a set of one white frame and one black frame is counted as a single "wave" (indicated by the two vertical orange lines marking the boundary of "one wave"). For this reason, the frequency displayed on the scope is half the actual refresh frequency, and the displayed period is twice the actual refresh period. In this case, 71.79 Hz indicates 71.79 sets of black-white transitions (2 frames) per second, for a total of 143.58 frames per second.
Verification by High-Speed Camera
This is a high-speed video of the blurbusters frame skipping test (https://testufo.com/frameskipping) shot with a Casio Exilim ZR100 at 1,000 FPS. Each frame of video represents 1 ms of real time. The video is played back at 30 FPS, meaning that every 1 second of video shows 30 ms of time. At 144 Hz, the display refreshes at intervals of 6.9444 ms. This means that we should see slightly more than 4 refreshes per second of video, which the video does show. This can also be verified more precisely by examining the video frame by frame and counting 7 frames between each refresh. We can also observe that the display is operating properly, without any frame skipping.