Qt: Fix HiDPI handling on X11/Windows#30399
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With X11, there is only ever a single scale, regardless of monitors as in Wayland, so it's always the highest scale (i.e., it's 2 if you have 1.5&1-scaled monitors). Thus we get no change events and don't update the internal scale. On Wayland though, as noted in the other issue from Qt devs, you only get the fractional scale after the first expose, so there's always at least one event there. In the older/pre-#30345 code path, in `showEvent`, we'd call `_update_screen` to set callbacks for its changes, and that _also_ called `_update_pixel_ratio`. In the new code, we don't have that initial call, and because Wayland always has at least one event at startup, it all seemed to work. So just add the `_update_pixel_ratio` call in the new code path as well. On X11, this will be the highest integer scale needed and never changes. On Wayland, this will also be the rounded-up integer scale, but if using fractional scaling, will change with a subsequent event to the correct one. This does cause a few extra changes on startup, but should be more consistent across platforms.
In fact, if on Wayland you had a integer scale, then the previous code would also be incorrectly scaled, since then you don't get that first event that happens with fractional scales. |
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Note, I do expect this to fix the problem on Windows as well, but I cannot test it. |
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Is this a X11/Windows issue only? I observe a degration on macOS as well. Window in the background is MPL 3.10.5, the one in the foreground is 3.10.3, both running on Qt/PyQt 6.9.1. 
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On the positive side: adding that 
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It's possibly also a problem on macOS; we just haven't had any reports of such, and I can't test it myself. |
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I can confirm that the issue also exists on macOS, bisects to #30345, and is fixed by this PR. I'll approve on that basis (and because the fix makes sense, of course). |
…399-on-v3.10.x Backport PR #30399 on branch v3.10.x (Qt: Fix HiDPI handling on X11/Windows)
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PR summary
With X11, there is only ever a single scale, regardless of monitors as in Wayland, so it's always the highest scale (i.e., it's 2 if you have 1.5&1-scaled monitors). Thus we get no change events and don't update the internal scale. On Wayland though, as noted in the other issue from Qt devs, you only get the fractional scale after the first expose, so there's always at least one event there.
In the older/pre-#30345 code path, in
showEvent, we'd call_update_screento set callbacks for its changes, and that also called_update_pixel_ratio. In the new code, we don't have that initial call, and because Wayland always has at least one event at startup, it all seemed to work.So just add the
_update_pixel_ratiocall in the new code path as well. On X11, this will be the highest integer scale needed and never changes. On Wayland, this will also be the rounded-up integer scale, but if using fractional scaling, will change with a subsequent event to the correct one. This does cause a few extra changes on startup, but should be more consistent across platforms.Fixes #30386
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